Cheers, season four, episode five: Diane's Nightmare. Toward the end of the nightmare, Diane dreams that Sam is a sensitive, urbane, pipe-smoking dandy. Partway through a preview of the opera he's been working on [which "combines the brooding intensity of Mahler with the pesky insouciance of Poulenc"], Sam is interrupted by Diane's advances:

"...put the keyboard away - let me be the instrument you play on."

To which he replies:

"Do you realize that you just ended that proposition with a preposition?"