Robert Spencer has written a book which asserts that Muhammad never existed. He has defended his theory in panel discussions with Muslims, who needless to say want to go jumpin jihad on him.
There’s a YouTube of Spencer and another guy debating with a couple of Muslims. It is shocking. Every single argument the Muslims use is circular. (The Quran has been perfectly preserved by God. This is proven by the fact that we now have it in its present form, which is perfect!) Spencer and the other guy repeatedly point out that each argument from the Muslims is circular. Again and again and again and again. The Muslims don’t seem to have the ability to come up with an argument that isn’t circular.