Marty, the future isn't written. It can be changed. You know that. Anyone can make their future whatever they want it to be.
11 seconds sound clip from the Back to the Future Part III movie soundboard.
You can hear this line at 01:09:34.345 in the DVD version of the movie.
Quote context
[...]
- Marty, I've made a decision. I'm not going with you tomorrow. I'm staying here.
- What are you talking about, Doc?
- There's no point in denying it. I'm in love with Clara.
- Oh, man!
- Doc, we don't belong here. Neither one of us.
- I can't let this one little photograph determine my entire destiny.
- I have to live my life according to what I believe is right in my heart.
- Doc, you're a scientist. So you tell me what's the right thing to do up here!
[...]