Kindly read the following webpage...
http://muhammadanism.org/Inquiries/Muha ... ad_sin.htm
It affirms what Alpha said...
But you are free to believe what you want to believe...
Much more could be written on the sinfulness of Muhammad, but the Qur'an and the ahadith should be an adequate testimony to a Muslim on this question. It is true that many Muslims don't believe the testimony of the Qur'an, ahadith, and seerah of Muhammad. Rather, they prefer an idealized and fictional prophet of their own imagination.