According to:
"Seeing Islam as Others Saw It: A Survey and Evaluation of Christian, Jewish and Zoroastrian Writings on Early Islam" the first note about Muhammed was made by Thomas the Presbyter in ca:640.
However, Thomas only refeers to him as "Muhammed of the Arabs" - and never states anything else characteristic about Muhammed. It basically only describes him as a leader of sorts in vauge terms.
Here is the book btw if anyone is intrested. Thomas the Presbyter is on page 120:
http://bookzz.org/book/1190569/0daf03
That's intresting since the dominant narrative has been Umayyad Caliphs who have tried to impose arabanization on other etnicities - and folks like the Persians trying to resist such assimilation processes.
Looks really intresting. And even more intresting - a library in my area has it! I'll definitively get my filthy, filthy paws on it soon!