Since Muhammad himself believed it was a demon, it's a cinch he didn't come up with that idea on his own. I thought it was his wife Khadijah who came up with the suggestion, but apparently through Waraqa bin Naufal, the occult Ebionite Priest who came up with the idea.
Ran across a couple of sites in a quick search. Didn't read them through.
"What has happened to me? I am afraid for myself! However, Khadija and on the advice of lbn Neafal her cousin managed to convince Muhammad that the spirit was Gabriel. But the spirit did not introduce itself as the angel Gabriel."
http://www.muslimhope.com/TestTheSpirit.htmhttps://www.thespiritofislam.com/books/6-islam-muhmmad-quran/59-muhammad-call.html
