- Drink up, Biff.
- Yeah, thanks.
- Ain't you coming?
- I'm reading.

4 seconds sound clip from the Back to the Future Part II movie soundboard.

You can hear this line at 01:16:41.589 in the Blu-ray version of the movie.

Quote context

[...]

- Well, thanks. Nice talking to you. Maybe we'll bump into each other some time again in the future.

- Or in the past.

(1946 Ford Tires Screeching)

- Doc, Doc! Come in!

- Where's that punk Calvin Klein, anyway?

- How am I supposed to know? I ain't his secretary.

- Well, go find him. He caused 300 bucks damage to my car, and I owe him a knuckle sandwich. Get going!

- Drink up, Biff.

- Yeah, thanks.

- Ain't you coming?

- I'm reading.

- Well, well, well, Mr. Tannen. How nice to see you here.

- Why, Mr. Strickland, it's nice to see you, sir.

- Is that liquor I smell, Tannen?

- I wouldn't know. I don't know what liquor smells like, 'cause I'm too young to drink it.

[...]