St John of Damascus is a very important witness to early Islam. He was born into a very privileged family in Damascus (his grandfather had been the administrator of the city at the time the Muslims took it) and he grew up and served in the court of the caliph. He was entirely familiar with Islam (a name it did not yet possess, apparently), and thus what he has to say about it, and the context in which he places it, is of great historical importance. For one thing, this is a single chapter in his work On Heresies, part of his larger work, The Fountain of Knowledge. Thus, during his lifetime, St John did not consider Islam to yet be a separate religion, but rather a Christian heresy.
HERE the complete text of what John of Damascus wrote on Islam.