Dumbledore is constantly saying this in fics. He doesn't have this line in the books, so I widened the net. 

 

Armando Dippet

• ‘My dear boy,’ said Dippet kindly, ‘I cannot possibly let you stay at school over the summer. Surely you want to go home for the holidays?’
• ‘Precisely,’ said the Headmaster. ‘My dear boy, you must see how foolish it would be of me to allow you to remain at the castle when term ends. Particularly in the light of the recent tragedy … the death of that poor little girl … You will be safer by far at your orphanage. As a matter of fact, the Ministry of Magic is even now talking about closing the school. We are no nearer locating the – er – source of all this unpleasantness …’

Elphias Doge

• ‘My dear boy! Arthur told me you were here, disguised … I am so glad, so honoured!’
• ‘Oh, please call me Elphias, dear boy.’

Albus Dumbledore

• ‘Calm yourself, dear boy, you are a little behind the times,’ said Dumbledore. ‘Quirrell does not have the Stone.’
• ‘Ah,’ said Dumbledore, smiling still more broadly. ‘That is the question, isn’t it? On the whole, dear boy, I think not.’
• ‘My dear boy, its remarkable effects were directed only at Voldemort, who had tampered so ill-advisedly with the deepest laws of magic. Only towards him was that wand abnormally powerful. Otherwise it was a wand like any other … though a good one, I am sure,’ Dumbledore finished kindly. 
• ‘My dear boy, I have no idea. This is, as they say, your party.’ 
• ‘My dear boy, I do,’ said Dumbledore, while his fellow pictures looked confused and curious. ‘A wise and courageous decision, but no less than I would have expected of you. Does anyone else know where it fell?’
• ‘Well, then, you must get on and do it, my dear boy,’ said Dumbledore softly.
• ‘My dear boy, let us have no more pretence about that. If you were going to kill me, you would have done it when you first Disarmed me, you would not have stopped for this pleasant chat about ways and means.’

Cornelius Fudge

• ‘Oh, my dear boy, we’re not going to punish you for a little thing like that!’ cried Fudge, waving his crumpet impatiently. ‘It was an accident! We don’t send people to Azkaban just for blowing up their aunts!’

Rubeus Hagrid

• Hagrid’s miserable face broke into a wide, watery smile. ‘Tha’s my boy … You show ’em, Harry, you show ’em. Beat ’em all.’

Gilderoy Lockhart

• ‘My dear boy,’ said Lockhart, straightening up and frowning at Harry. ‘Do use your common sense. My books wouldn’t have sold half as well if people didn’t think I’d done all those things. No one wants to read about some ugly old Armenian warlock, even if he did save a village from werewolves. He’d look dreadful on the front cover. No dress sense at all. And the witch who banished the Bandon Banshee had a hairy chin. I mean, come on …’

Sir Nicholas de Mimsy-Porpington

• ‘Harry! My dear boy!’
• ‘My dear boy! Harry Potter, at my Deathday Party! And,’ he hesitated, looking excited, ‘do you think you could possibly mention to Sir Patrick how very frightening and impressive you find me?’

Garrick Ollivander

• ‘My dear boy.’ Ollivander’s voice was feeble. ‘You rescued us. I thought we would die in that place. I can never thank you … never thank you … enough.’

Tom Riddle

• ‘Ah, yes, I remember,’ said Voldemort, looking down at Neville, who was struggling back to his feet, unarmed and unprotected, standing in the no-man’s-land between the survivors and the Death Eaters. ‘But you are a pure-blood, aren’t you, my brave boy?’ Voldemort asked Neville, who stood facing him, his empty hands curled in fists.

Rufus Scrimgeour

• ‘My dear boy … even Dumbledore cannot return from the –’

Horace Slughorn

• ‘Harry, m’boy!’ said Slughorn, jumping up at the sight of him so that his great velvet-covered belly seemed to fill all the remaining space in the compartment. His shiny bald head and great silver moustache gleamed as brightly in the sunlight as the golden buttons on his waistcoat. 
• ‘Harry, m’boy?’
• ‘Ah yes, Professor McGonagall did mention … not to worry, my dear boy, not to worry at all. You can use ingredients from the store cupboard today, and I’m sure we can lend you some scales, and we’ve got a small stock of old books here, they’ll do until you can write to Flourish and Blotts …’
• ‘Harry, m’boy!’ said a booming voice from behind them.
• ‘Harry, that’s three of my little suppers you’ve missed now!’ said Slughorn, poking him genially in the chest. ‘It won’t do, m’boy, I’m determined to have you! Miss Granger loves them, don’t you?’
• ‘Harry, m’boy!’ boomed Slughorn, almost as soon as Harry and Luna had squeezed in through the door. ‘Come in, come in, so many people I’d like you to meet!’
• ‘Ask away, then, m’boy, ask away …’
• ‘Don’t mention it, m’boy, don’t mention it,’ said Slughorn, as Ron collapsed into a nearby armchair, looking devastated.
• ‘Euphoria, I take it? And what’s that I smell? Mmmm … you’ve added just a sprig of peppermint, haven’t you? Unorthodox, but what a stroke of inspiration, Harry. Of course, that would tend to counterbalance the occasional side-effects of excessive singing and nose-tweaking … I really don’t know where you get these brainwaves, my boy … unless –’
• ‘That’s enough!’ said Slughorn suddenly, raising a shaking hand. ‘Really, my dear boy, enough … I’m an old man … I don’t need to hear … I don’t want to hear …’
• ‘But then … my dear boy … you’re asking a great deal … you’re asking me, in fact, to aid you in your attempt to destroy –’
• ‘Harry!’ he panted, massaging his immense chest beneath his emerald-green silk pyjamas. ‘My dear boy … what a surprise … Minerva, do please explain … Severus … what …?’

Hepzibah Smith

• ‘Lean in a little, dear boy, so you can see … of course, Burke knows I’ve got this one, I bought it from him, and I daresay he’d love to get it back when I’m gone …’

Sybill Trelawney 

• ‘My dear boy – my poor dear boy – no – it is kinder not to say – no – don’t ask me …’
• ‘The Grim, my dear, the Grim!’ cried Professor Trelawney, who looked shocked that Harry hadn’t understood. ‘The giant, spectral dog that haunts churchyards! My dear boy, it is an omen – the worst omen – of death!’
• ‘Indeed!’ whispered Professor Trelawney, scribbling keenly on the parchment perched upon her knees. ‘My boy, you may well be seeing the outcome of poor Hagrid’s trouble with the Ministry of Magic! Look closer … does the Hippogriff appear to … have its head?’
• ‘I’m so sorry, dear boy,’ she said dreamily. ‘The heat of the day, you know … I drifted off for a moment …’
• ‘My dear boy!’ she said in a very carrying whisper. ‘The rumours! The stories! The Chosen One! Of course, I have known for a very long time … the omens were never good, Harry … but why have you not returned to Divination? For you, of all people, the subject is of the utmost importance!’

Fred Weasley

• ‘Harry!’ said Fred, elbowing Percy out of the way and bowing deeply. ‘Simply splendid to see you, old boy –’

Oliver Wood

• ‘That’s my boy!’ Wood kept yelling. Alicia, Angelina and Katie had all kissed Harry, and Fred had him in a grip so tight Harry felt as though his head would come off. In complete disarray, the team managed to make its way back to the ground. Harry got off his broom and looked up to see a gaggle of Gryffindor supporters sprinting onto the pitch, Ron in the lead. Before he knew it, he had been engulfed by the cheering crowd.

Eldred Worple

• ‘Just as modest as Horace described!’ said Worple. ‘But seriously –’ his manner changed; it became suddenly businesslike, ‘I would be delighted to write it myself – people are craving to know more about you, dear boy, craving! If you were prepared to grant me a few interviews, say in four- or five-hour sessions, why, we could have the book finished within months. And all with very little effort on your part, I assure you – ask Sanguini here if it isn’t quite – Sanguini, stay here!’ added Worple, suddenly stern, for the vampire had been edging towards the nearby group of girls, a rather hungry look in his eye. ‘Here, have a pasty,’ said Worple, seizing one from a passing elf and stuffing it into Sanguini’s hand before turning his attention back to Harry.
• ‘My dear boy, the gold you could make, you have no idea –’

If you’ve ever wanted a list of nightwear in Harry Potter (LOL) I can deliver:


Archie Aymslowe

GoF: Ludo Bagman was easily the most noticeable person Harry had seen so far, even including old Archie in his flowered nightdress.

Moody!Barty Crouch

GoF: Snape stopped talking very abruptly. He and Filch both looked down at the foot of the stairs. Harry saw Mad-Eye Moody limp into sight through the narrow gap between their heads. Moody was wearing his old travelling cloak over his nightshirt and leaning on his staff as usual.

Albus Dumbledore

CoS: Next moment, Dumbledore was backing into the dormitory, wearing a long woolly dressing gown and a nightcap. He was carrying one end of what looked like a statue. Professor McGonagall appeared a second later, carrying its feet. Together, they heaved it onto a bed.

OotP: Dumbledore was sitting in a high-backed chair behind his desk; he leaned forward into the pool of candlelight illuminating the papers laid out before him. He was wearing a magnificently embroidered purple and gold dressing gown over a snowy white nightshirt, but seemed wide-awake, his penetrating light blue eyes fixed intently upon Professor McGonagall.

Dudley Dursley

HBP: Dudley had that moment peered round the living room door. His large, blond head rising out of the stripy collar of his pyjamas looked oddly disembodied, his mouth gaping in astonishment and fear. Dumbledore waited a moment or two, apparently to see whether any of the Dursleys were going to say anything, but as the silence stretched on he smiled.

Petunia Dursley

HBP: The kitchen door had opened, and there stood Harry’s aunt, wearing rubber gloves and a housecoat over her nightdress, clearly halfway through her usual pre- bedtime wipe-down of all the kitchen surfaces. Her rather horsey face registered nothing but shock.

Vernon Dursley

HBP: Vernon Dursley, whose moustache was quite as bushy as Dumbledore’s, though black, and who was wearing a puce dressing gown, was staring at the visitor as though he could not believe his tiny eyes.

Lily Evans

DH: It was nighttime. Lily, who was wearing a dressing gown, stood with her arms folded in front of the portrait of the Fat Lady, at the entrance to Gryffindor Tower.

Argus Filch

GoF: sick with apprehension, he watched Filch drawing nearer and nearer in his old flannel dressing-gown – he tried desperately to pull his trapped leg free, but it merely sank a few more inches – any second now, Filch was going to spot the map or walk right into him –

Cornelius Fudge

OotP: “I know, Williamson, I know, I saw him too!” gibbered Fudge, who was wearing pyjamas under his pinstriped cloak and was gasping as though he had just run miles. “Merlin’s beard — here — here! — in the Ministry of Magic! — great heavens above — it doesn’t seem possible — my word — how can this be?”

Gilderoy Lockhart

OotP: Their ex-Defence Against the Dark Arts teacher pushed open the doors and moved towards them, wearing a long lilac dressing gown.

Alice Longbottom

OotP: Neville’s mother had come edging down the ward in her nightdress.

Xenophilius Lovegood

DH: Barely ten seconds passed, then the door was flung open and there stood Xenophilius Lovegood, barefoot and wearing what appeared to be a stained nightshirt. His long, white, candyfloss hair was dirty and unkempt. Xenophilius had been positively dapper at Bill and Fleur’s wedding by comparison.

Minerva McGonagall

PoA: The Gryffindor party ended only when Professor McGonagall turned up in her tartan dressing gown and hair net at one in the morning, to insist that they all go to bed. Harry and Ron climbed the stairs to their dormitory, still discussing the match.

OotP: Professor McGonagall came hurrying into the dormitory in her tartan dressing gown, her glasses perched lopsidedly on the bridge of her bony nose.

DH: “Oh, are we allowed to say the name now?” asked Luna with an air of interest, pulling off the Invisibility Cloak. This appearance of a second outlaw seemed to overwhelm Professor McGonagall, who staggered backward and fell into a nearby chair, clutching at the neck of her old tartan dressing gown.

DH: Along the corridors they raced, and one by one the Patronuses left them; Professor McGonagall’s tartan dressing gown rustled over the floor, and Harry and Luna jogged behind her under the Cloak.

Poppy Pomfrey

CoS: “Get Madam Pomfrey,” whispered Dumbledore, and Professor McGonagall hurried past the end of Harry’s bed out of sight. Harry lay quite still, pretending to be asleep. He heard urgent voices, and then Professor McGonagall swept back into view, closely followed by Madam Pomfrey, who was pulling a cardigan on over her nightdress. He heard a sharp intake of breath.

HBP: After a while Madam Pomfrey came out of her office, this time wearing a thick dressing gown.

Harry Potter

GoF: Then, when the deep pool was full of hot water, foam, and bubbles, which took a very short time considering its size, Harry turned off all the taps, pulled off his pyjamas, slippers, and dressing gown, and slid into the water.

Baby!Harry, DH: They had not drawn the curtains; he saw them quite clearly in their little sitting room, the tall black-haired man in his glasses, making puffs of coloured smoke erupt from his wand for the amusement of the small black-haired boy in his blue pyjamas. The child was laughing and trying to catch the smoke, to grab it in his small fist. ...

Mrs Roberts

GoF: The floating people were suddenly illuminated as they passed over a burning tent, and Harry recognised one of them – Mr Roberts, the campsite manager. The other three looked as though they might be his wife and children. One of the marchers below flipped Mrs Roberts upside-down with his wand; her nightdress fell down to reveal voluminous drawers; she struggled to cover herself up as the crowd below her screeched and hooted with glee.

Madam Rosmerta

HBP: His heart leapt: somebody had seen, somebody knew they needed help – and looking around he saw Madam Rosmerta scurrying down the dark street towards them on high-heeled, fluffy slippers, wearing a silk dressing-gown embroidered with dragons.

Horace Slughorn

HBP: Harry had been slightly worried that Slughorn might be at breakfast, but he answered his office door at the first knock, wearing a green velvet dressing gown and matching nightcap and looking rather bleary-eyed.

DH: “Harry!” he panted, massaging his immense chest beneath his emerald-green silk pyjamas. “My dear boy ... what a surprise ... Minerva, do please explain. ... Severus ... what ... ?”

Severus Snape

GoF: Filch stopped a few steps below Harry and turned. At the foot of the stairs stood the only person who could make Harry’s situation worse: Snape. He was wearing a long grey nightshirt and he looked livid.

Crowds of Students

DH: They parted at the top of the next staircase, Harry and Luna running back toward the concealed entrance to the Room of Requirement. As they ran, they met crowds of students, most wearing travelling cloaks over their pyjamas, being shepherded down to the Great Hall by teachers and prefects.

Dean Thomas

OotP: “Hey, Harry,” said Dean, who was putting on a pair of pyjamas in the West Ham colours. “Good holiday?”

Tom the Barman

PoA: Tom the innkeeper reappeared, wearing an apron over his nightshirt and bearing a tray of tea and crumpets. He placed the tray on a table between Fudge and Harry and left the parlour, closing the door behind him.

Arthur Weasley

OotP: The kitchen door opened and the entire Weasley family, plus Hermione, came inside, all looking very happy, with Mr Weasley walking proudly in their midst dressed in a pair of striped pyjamas covered by a mackintosh.

OotP: Mrs Weasley did not seem entirely satisfied with Mr Weasley’s answer. As her husband leaned over to shake Harry’s hand, she peered at the bandaging under his nightshirt.

Ginny Weasley

CoS: At that moment there was a diversion in the form of a small, red-headed figure in a long nightdress, who appeared in the kitchen, gave a small squeal, and ran out again.

Molly Weasley

OotP: All were fully dressed except Mrs Weasley, who was wearing a quilted purple dressing gown. She leapt to her feet the moment Harry entered.

HBP: There stood Mrs Weasley, short, plump and wearing an old green dressing-gown.

Ron Weasley

PS: Harry stepped aside, but with Ron in front of the mirror, he couldn’t see his family any more, just Ron in his paisley pyjamas.

GoF: Dressed in his maroon paisley pyjamas, Ron stopped dead facing Harry across the room, and looked around.

GoF: He strode across the room towards the stairs; he half expected Ron to stop him, he would even have liked Ron to throw a punch at him, but Ron just stood there in his too small pyjamas, and Harry, having stormed upstairs, lay awake in bed fuming for a long time afterwards, and didn’t hear him come up to bed.

OotP: “We can’t let them out to hunt every night,” Ron explained as he pulled on his maroon pyjamas.

OotP: “Ron, I’ll have to get you more pyjamas, these are at least six inches too short, I can’t believe how fast you’re growing ... what colour would you like?”

DH: “I still haven’t ruled it out,” came Hermione’s muffled voice from beneath her blankets, but Harry saw Ron smiling slightly as he pulled his maroon pyjamas out of his rucksack.