You sent me a letter.

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

You can hear this line at 00:07:14.318 in the DVD version of the movie.

Quote context

[...]

- Look, I came back to 1955 again with you, the you from 1985, 'cause we had to get a book from Biff!

- So once I got the book back, you, that is the you from 1985, were in the DeLorean, and it got struck by lightning, and you got sent back to 1885!

- 1885?

- It's a very interesting story, future boy, but there's just one little thing that doesn't make sense.

- If the me of the future is now in the past, how could you possibly know about it?

- You sent me a letter.

- Dear Marty, if my calculations are correct, you will receive this letter immediately after you saw the DeLorean struck by lightning.

- First, let me assure you that I'm alive and well.

- I've been living happily these past eight months in the year 1885.

- The lightning bolt that hit the DeLorean caused a gigawatt overload which scrambled the time circuits, activated the Flux Capacitor, and sent me back to 1885.

[...]