That way, we don't risk anyone else stealing the time machine, and I won't risk accidentally running into my other self.

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

You can hear this line at 01:03:18.303 in the Blu-ray version of the movie.

Quote context

[...]

- Once Old Biff is gone, grab the almanac any way that you can.

- Remember, both of our futures depend on this.

- You don't have to remind me of that, Doc.

- Here's some binoculars and a walkie-talkie so we can keep in contact.

- I'll stay here and try to repair the short in the time circuit.

- That way, we don't risk anyone else stealing the time machine, and I won't risk accidentally running into my other self.

- Other self?

- Yes. There are now two of me here, and there are two of you here.

- The other me is the Dr. Emmett Brown from 1955, the younger me that helps the other you get back to 1985.

- Remember the lightning bolt at the clock tower?

- Yeah.

- That event doesn't happen until tonight, so you must be very careful not to run into your other self.

[...]