However this claim cannot be substantiated, since precise records were not kept after Tannen shot a newspaper editor who printed an unfavorable story about him in 1884. That's why we can't find anything.
10 seconds sound clip from the Back to the Future Part III movie soundboard.
You can hear this line at 00:14:48.146 in the DVD version of the movie.
Quote context
[...]
- Shot in the back by Buford Tannen over a matter of $80?
- What kind of a future do you call that?
- Does it mention me? Am I one of the 12?
- Look. William McFly and family. Your relatives?
- My great-grandfather's name was William. That's him. Good-looking guy.
[...]