I wouldn't do that if I was you.
4 seconds sound clip from the Dark Command (1940) movie soundboard.
You can hear this line at 00:17:48 in the Blu-ray version of the movie.
Quote context
[...]
- And, once again, thank you very much.
- She went across the street to buy a spool of thread or something.
- I wouldn't do that if I was you.
- She always gets a little fidgety when she's buying a spool of thread or something.
- Dad, you tell him about me goin' to be a cowboy. Go ahead and tell him.
- That's the only time I was ever in jail, and it was the other fella's fault. I was drunk.
- Drinking, of course, constituting no crime.
[...]