(Flashback to Peter in Willy Wonka's factory)
Wonka: I'll ask you one more time, are you sure you didn't eat anything in my factory?
Peter: (appears as a giant blueberry) No.
Wonka: I'm just asking.
Peter: Are you calling me a liar?
Wonka: No, I'm just saying -
Peter: Hey, shut up Wonka! [edit]

I see it first used in a family guy episode, 1999. I don't see earlier usages (after a quick poke around google results from 1990-2000.

http://www.tv.com/family-guy/i-never-met-the-dead-man/episode/17635/trivia.html