If I told you, it might not come true.
4 seconds sound clip from the It's a Wonderful Life (1946) movie soundboard.
You can hear this line at 00:27:06 in the Blu-ray version of the movie.
Quote context
[...]
- Buffalo gals can't you come out tonight? Can't you come out tonight? Can't you come out tonight? (2)
- Buffalo gals, can't you come out tonight and dance by the light of the moon? (2)
- What did you wish when you threw that rock?
- If I told you, it might not come true.
- Hey, that's a pretty good idea. I'll give you the moon, Mary. I'll take it.
- Well, then you could swallow it, and it would all dissolve, see?
- And the moonbeams would shoot out of your fingers and your toes and the ends of your hair.
[...]