That's widely believed but the Wikipedia says that it's likely a misattribution and provides a couple of twists:

I didn't get it from Wikipedia, as Wikipedia can be edited. I don't trust it.

Surely, I'm not saying it's correct. Anyway it's argued upon language style, the case is whether Franklin actually wrote it or a contemporary person heard it from him and registered it or something.

My favourite variant is the last one, the one with the "must".

Sweet, sweet off topic...