"Wow - we're identical!" (Fred and George)
"I dunno, though, I think I'm still better-looking" (Fred)

Said in the chapter 'the Seven Potters' in DH, when they are all drinking the Polyjuice Potion to look exactly like Harry.

"I'm not Fred, I'm George. Honestly, woman, you call yourself our mother? Can't you tell I'm George?" (Fred)

Chapter 6 in PS - "Journey from Platform 9 3/4" when Molly tell Fred to go into the Platform.

Fred: That's not what he said!
George: Would you like us to clean out your ears for you? (pulls out long lethal looking metal instrument)
Fred: Or any part of your body, really. We're not fussy where we stick this!

Said on Order of the Phoenix during the D.A meeting in Hog's Head by Fred and George to Zacharias Smith.

Fred: You know what? I don't think we are. George, I think we've outgrown full-time education.
George: Yeah, I've been feeling the same way myself.
Fred: Time to test our talents in the real world, do you reckon?
George: Definitely.
Fred: We won't be seeing you.
George: Yeah, don't bother to keep in touch.
Fred: If anyone fancies buying a Portable Swamp, as demonstrated upstairs, come to number ninety-three, Diagon alley- Weasleys' Wizarding Wheezes. Our new premises!
George: Special discounts to Hogwarts students who swear they're going to use our products to get rid of this old bat.
Fred: Give her hell from us, Peeves.

Also in OotP Chapter is Careers Advice, during Fred and George's flight from Hogwarts.

"You haven't got a letter on yours," George observed. " I suppose she thinks you don't forget your name. But we're not stupid-- we know we're called Gred and Forge."

Harry Potter and The Sorcerer's Stone, Chapter Twelve The Mirror of Erised, Page 202, Christmas morning.