Marty, you have to wear the boots. You can't wear those futuristic things back in 1885. You shouldn't even be wearing them here in 1955.
7 seconds sound clip from the Back to the Future Part III movie soundboard.
You can hear this line at 00:16:22.073 in the DVD version of the movie.
Quote context
[...]
- Yeah, everything except the boots, Doc. They're kind of tight.
- I don't know. Are you sure this stuff is authentic?
- Haven't you ever seen a Western?
- Yeah, I have, Doc. And Clint Eastwood never wore anything like this.
- That's right. You haven't heard of him yet.
- All right, Doc. Look, as soon as I get there, I'll put them on. I promise.
- Okay. I think we're about ready. I put gas in the tank.
- Your future clothes are packed.
- Just in case, fresh batteries for your walkie-talkies.
- Oh! What about that floating device?
[...]