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. In any case, he mentions several suras of the Qur’an by name, and refers most interestingly to one which is no longer extant. St John, in this work, as characteristically, pulls no punches. Enjoy.
And there is also the up until now strong and people-deceiving superstition of the Ishmaelites, being the forerunner of Antichrist. And it is born from Ishmael, who was born from Hagar to Abraham, from which they are called Hagarenes and Ishmaelites. And they call them Saracens, as from Σαρρας κενοι (those empty of Sarah), because of what was said by Hagar to the angel: “Sarah has sent me away empty.” So then, these were idolaters and reverenced the morning star and Aphrodite, who they indeed named Khabar in their own language, which means great. Therefore, until the time of Heraclius, they were plainly idolaters. From that time and until now came up among them a false prophet called Mamed, who, having encountered the Old and New Testament, as it seems, having conversed with an Arian monk, he put together his own heresy. And under the pretext of seeming pious, attracting (?) people, he reported that a book was sent down to him from heaven by God. Therefore some of the compositions written by him in a book, worthy of laughter, which he handed down to them as an object of reverence.