Sorry the other citation is:
http://skeptically.org/enlightenment/id3.html
Early written Arabic didn't emerge until around the 9th century. Ishaq however, is the earliest account of the life of Muhammad, myth or real, and even that doesn't exist in it's entirety, much of it has been edited out by later "scholars" because it was too embarasing. So we have to rely on Tabari,and Hisham and assume they included all of Ishaq's work, which we know Hisham didn't.. Bukari and Muslim (his student) came much later and added their own work, edited some of the earlier. For a supposedly recent "prohet" nothing survives that can prove his existence, which is odd.
There certainly wasn't a "koran" handed down around in Muhammads lifetime, as Muslims try to insist. Nobody even heard of Moe while he was alive, only later after Muhammad died and Muhammadans began slaughtering their way across the ME did they become aware of Islam.