The De Boor edition of Theophanes is online at Google Books, although as it comes from a UK library, UK people are not allowed to view it (!). But I’ve been looking at the Bonn edition, which comes with Latin translation and so easier to thumb through. P. 503 is the first mention of Mohammed, in a list of lengths of reigns (plainly derived from some of the tables of years that we find in Eusebius, and Jacob of Edessa):
Of the Arabs, leader Muamed for nine years.
Paging down, I look for more. The same appears on p. 504 and 506! On p.511 (AD 622) it begins seriously:
Hoc anno Muamed Saracenorum dux et pseudopropheta, Abubachr cognato suo successore designato, mortuus est, ex quo omnes estimare.
In this year Muamed, general of the Saracens and false-prophet, having designated Abu-Bakr as his successor, died, as everyone calculates.
He then says that the Jews went over to Mohammed, believing him to be the expected Messiah. Then the story of Mohammed and his teaching runs on to p.514.
Anyone fancy translating the Greek?