I have never heard that claim. There is, however, a firm tradition among Arab Christians that Mohammed started as a Christian missionary from the Nestorian Church of the East, then went rogue and set up as a warlord. There is some support for this view to be found in recent textual criticism of the Qu’ran, some passages of which are nonsense in Arabic, but perfectly good East Syriac, with specific textual parallels in Assyrian Christian texts.
It is my own observation on the matter, and in support of this tradition, that the bishop of what is now Kirkuk at the time Mohammed began his activities was named Mar Gabriel. There was an ancient custom, still preserved in one of the titles of the Orthodox Bishop of Bosra-Haran, the “Angel of Haran”, and found in the Apocalypse of St. John, of referring to the bishop as the “angel” of his local church. It is quite possible that when he began composing the Qu’ran, Mohammed’s assertions that “the angel Gabriel” told him things, were not delusional claims that the bodiless one who had made the Annunciation to the Theotokos and Ever-Virgin Mary was speaking to him, but simply reporting that he got what he was writing from his bishop, Mar Gabriel.
This might be related,
I recollection was it had something to do with Syria, maybe it was just Assyrian branch of the church that I am remembering.
So maybe it was Assyrian Mission then, but I remember that there was a letter spoke of being written but not quoted from directly. It was just a summary of the content of a letter written speaking about Muhammad into his background and behavior.
It wasn’t some vague tradition. It was a report recorded about a letter that was received about a 50 so years before it was recorded there and about the event that preceded the letter 50-80 years.
So it was about 100-120 years before it was recorded in the text.
Although the exact contents escape me now, I recall that this information confirmed to me that Muhammad was a absolutely fake and so was the religion of Islam.
It was so stark, that I believed that even a non-Christian would reach the same conclusion.
I remember the assignment in Catholic school was to study this because at the time, there was this idea (still is today) that different religions all referred to the same God.