It was in the episode "Heartbreak City"
A quote from a different episode:
Piper: Prue, wh-what are you doing in here?
Leo: Piper, can you hand me a towel? (Leo pulls across the shower curtain and sees Prue.) Prue!
(Prue's eyes widen.)
Prue: Leo. (Piper throws him a towel.) Nice orbs.
Piper: Bye. (Prue continues to look at Leo.) Bye.