- And where might that be?
- I can't tell you.

5 seconds sound clip from the Back to the Future Part III movie soundboard.

You can hear this line at 01:12:55.101 in the DVD version of the movie.

Quote context

[...]

- Emmett, won't you come in?

- No, I better not. I...

- What's wrong?

- I've come to say goodbye.

- Goodbye? Well, where are you going?

- I'm going away. And well, I'm afraid I'll never see you again.

- Emmett!

- Clara, I want you to know that I care about you deeply.

- But I've realized that I don't belong here, and I have to go back where I came from.

- And where might that be?

- I can't tell you.

- Well, wherever you're going, take me with you.

- I can't, Clara. I wish it didn't have to be this way, but just believe me when I tell you that I'll never forget you and that I love you.

- I don't understand what you're trying to say.

- Clara, I don't think there's any way that you can understand it.

[...]