It was in the ninth episode of ninth season of Friends. Here is the transcription:
[Scene: Ross and Rachel's apartment. Ross is in the living room covering Emma. Rachel enters wearing a sexy dress.]
ROSS: Wow! (pause) Wow, You look . . . uh . . .It's just, ah . . . That dress . . . uh . . .
RACHEL: Well, I hope the ends of these sentences are good.
ROSS: Well, well, they're good. It's been a while since I've seen you like this. You, you clean up good.