Big Battle Postfix No.1 - Who Confirmed Muhammads Existance?

This is a conversation that I had with Abdul Razaaq, after the big battle that we had discussing the Bible and Quran at Speakers Corner. As per usual he just wanted to attack the Bible, while ignoring the history of his belief, so I tried to show him that Jesus in the Bible had evidence of His existence from non Christian sources, while Muslims do not seem to have the equivalent for Muhammad. As expected, he did not do a very good job of explaining Islam's history, because this is not something that Muslims are taught.