Just a heresy though? That's really stretching it IMO.
Your's is a humble opinion compared to St. John of Damascus. I am going to go with his opinion as he states in his work On Heresies
“There is also the superstition of the Ishmaelites which to this day prevails and keeps people in error, being a forerunner of the Antichrist. They are descended from Ishmael, [who] was born to Abraham of Hagar, and for this reason they are called both Hagarenes and Ishmaelites. They are also called Saracens, which is derived from Sarras kenoi, or destitute of Sara, because of what Hagar said to the angel: ‘Sara hath sent me away destitute.’ These used to be idolaters and worshiped the morning star and Aphrodite, whom in their own language they called Khabár, which means great. And so down to the time of Heraclius they were very great idolaters. From that time to the present a false prophet named Mohammed has appeared in their midst. This man, after having chanced upon the Old and New Testaments and likewise, it seems, having conversed with an Arian monk, devised his own heresy.
Then, having insinuated himself into the good graces of the people by a show of seeming piety, he gave out that a certain book had been sent down to him from heaven. He had set down some ridiculous compositions in this book of his and he gave it to them as an object of veneration.”