There are however some hints at Mohammed in other, Jewish and Christian texts..... There is a document called the "Doctrina iacobi" originally in Aramaic, which dates from 634, two years after Mohammed's assumed death. It is a discussion between two Jewish friends, one of whom became a Christian, which caused some upset and lengthy discussions. In amongst all of that, there is this passage:
“When the candidatus [i.e. Sergius, a member of the Byzantine imperial army in Palestine] was killed by the Saracens, I was at Caesarea and I set off by boat to Sykamina. People were saying "the candidatus has been killed," and we Jews were overjoyed. And they were saying that the prophet had appeared, coming with the Saracens, and that he was proclaiming the advent of the anointed one, the Christ who was to come. I, having arrived at Sykamina, stopped by a certain old man well-versed in scriptures, and I said to him: "What can you tell me about the prophet who has appeared with the Saracens?" He replied, groaning deeply: "He is false, for the prophets do not come armed with a sword. Truly they are works of anarchy being committed today and I fear that the first Christ to come, whom the Christians worship, was the one sent by God and we instead are preparing to receive the Antichrist. Indeed, Isaiah said that the Jews would retain a perverted and hardened heart until all the earth should be devastated. But you go, master Abraham, and find out about the prophet who has appeared." So I, Abraham, inquired and heard from those who had met him that there was no truth to be found in the so-called prophet, only the shedding of men's blood. He says also that he has the keys of paradise, which is incredible [i.e. not credible]”.
This is a most fascinating passage, as it clearly speaks of a "prophet" from the "Saracens" (i.e. The Arabs) who was a fighter and he also spoke of a "Messiah". Given the date, it is almost inescapable that this is talking about Mohammed.
He was not accepted by the Jews and Christians, on account of his violence, the source says.
But is this the "Mohammed" of Islamic traditions? The first interesting part is that Mohammed is placed at the LEVANT. The narrator is in what today is Northern Central Israel and he is embarking on a journey to Cyprus, when he heard about the "prophet". The Jews (are you listening, eagle? JEWS in Palestine in 634) were happy at first about the attack on the Byzantines, but did not think much of the "prophet" claim.
It also worth noting that this "prophet" claimed had the "keys of paradise" which is a reference to Matthew's gospel and Peter. This testimony makes Mohammed essentially a Christian heretic who claimed to be the successor of the apostle Peter and was sent to prepare for the day of judgement, or the (second?) arrival of the Messiah.
So, the earliest sources portray Mohammed as a sort of "prophet" of a vaguely Christian variety, but with an army and a sword.... he "morphed" later into the "seal of the prophets" and Jewish and Christian reference points were shed.
So we can say that while Mohammed was most likely a historical figure, but the Islamic version of his life is far removed from the real Mohammed.