You look in your papers, ladies and gentleman, and you'll find Miss Mitchell's signed confession that she was the one that wrote it.
7 seconds sound clip from the Meet John Doe (1941) movie soundboard.
You can hear this line at 01:45:16 in the DVD version of the movie.
Quote context
[...]
- Were you paid or weren't you?
- And the suicide note. You didn't write that either.
- What difference does that make?
- Did you write it or didn't you?
- No, I didn't. You bet your life you didn't.
- Listen, folks, it's a fact that I didn't write the letter...
- Now you see, he admits it. You're a fake, John Doe.
- Speak up, John. We believe you.
[...]