I'll believe that when I see it.
4 seconds sound clip from the Grease (1978) movie soundboard.
You can hear this line at 00:45:04 in the Blu-ray version of the movie.
Quote context
[...]
- Simple's right. Too bad his brains are in his biceps.
- Jealous? Oh, come on, Sandy. Don't make me laugh.
- Oh. Oh, come on. I can run circles around those jerks.
- I'll believe that when I see it.
- Let's start with the first rule. Cutting down to two packs a day.
- Now what sort of athletics are you interested in? Do you like the rings?
- Yeah. I installed some rings just a couple of weeks ago.
- Well, the first thing we have to do is, you have to change.
[...]