According to Chabadpedia, the following comprises the signature of Rabbi Yisrael ben Eliezer, the Baal Shem Tov:

Is this actually the Baal Shem Tov's signature? Where was it copied from? The article does not say, and I am curious as to whether or not we know for certain that this is his.

Note: I asked this question on Mi Yodeya, but was told that it falls outside the parameters of that Stack Exchange. This is a shame, since it strikes me as the most appropriate place to ask a question of this nature. For those who are interested, here is a good article on the man in question. It makes no reference to his signature.