Moony, Wormtail, Padfoot, and Prongs,” sighed George, patting the heading of the map. “We owe them so much.”
“Noble men, working tirelessly to help a new generation of lawbreakers,” said Fred solemnly.
“Right,” said George briskly. “Don’t forget to wipe it after you’ve used it —”
“— or anyone can read it,” Fred said warningly.[Prisoner of Azkaban, Chapter 10]

You’re mental,” said George, trying to push it back at Harry.
“No, I’m not,” said Harry. “You take it, and get inventing. It’s for the joke shop.”
“He is mental,” Fred said in an almost awed voice. [Goblet of Fire]

Hey, look — Harry’s got a Weasley sweater, too!”

Fred and George were wearing blue sweaters, one with a large yellow F on it, the other a G.

“Harry’s is better than ours, though,” said Fred, holding up Harry’s sweater. “She obviously makes more of an effort if you’re not family.

Hermione drew herself to her full height; her eyes were narrowed and her hair seemed to crackle with electricity.
"No," she said, her voice quivering with anger, "but I will write to your mother.

Fred and George exchanged looks.
"You don't mind if we don't kiss you, do you, Ron?" said Fred in a falsely anxious voice.
"We could curtsy, if you like," said George.
"Oh, shut up," said Ron, scowling at them.

Well, you can't break an Unbreakable Vow...."
"I'd worked that much out for myself, funnily enough. What happens if you break it, then?"
"You die," said Ron simply. "Fred and George tried to get me to make one when I was about five. I nearly did too, I was holding hands with Fred and everything when Dad found us. He went mental," said Ron, with a reminiscent gleam in his eyes. "Only time I've ever seen Dad as angry as Mum. Fred reckons his left buttock has never been the same since."
“Yeah, well, passing over Fred’s left buttock —”
"I beg your pardon?" said Fred's voice as the twins entered the kitchen.