What a strange question. Everybody assumes that Muhammad existed. Does not Islam affirm his existence? Why would anyone question it? Strangely enough there are many questions about Muhammad, the Qur’an, and the hadiths (traditions) that arose about Muhammad that led some scholars and researchers to the conclusion that Muhammad really did not exist. What is the cause of this doubt?
A recent book by Robert Spencer has the title, Did Muhammad Exist? The book is well-researched and deals with many historical issues. He describes the “canonical” story, that is, the common story told by Muslims, of Muhammad and then deals with the problems of supporting the story. The conclusion is that there is little to support the Muslim claims concerning the existence of Muhammad historically.
Source: Review by Dallas M. Roark, answeringislam.org