Did Muhammad exist? No. And neither did the Koran for that matter, at least not in the form or with the status ascribed to it by Muslims. These are the two major conclusions of a vigorous stream of revisionist scholarship that has struggled for many decades to gain a hearing within the critically indolent field of Islamic Studies.
Source: Quadrant Online magazine article.
Check out Did Muhammad Exist? An Inquiry into Islam’s Obscure Origins, by Robert Spencer (Intercollegiate Studies Institute, 2012), 272 pages, US$39.95. You can buy it here used, usually for US$5.00 or less. Currently it's under $3.00 for a beat-up copy.
The research of Muhammad Sven Kalisch, Muslim convert and Germany's first professor of Islamic theology, is causing outrage among Muslims. His theory: The Prophet Muhammad probably never existed.
Source: Andrew Higgins of the Wall Street Journal. Here is the article.