The problem I see with your question over on the Christianity stack, and IMHO it would be a problem on any SE site, is that merely by being phrased the way it is, it implies that there's a good reason to believe it might be true, even though as near as I can tell you actually have no real reason to believe it might be true.
Questions that seem to start with a preferred answer tend to annoy people. Couple that with the fact that you are talking about the central figure in one of the world's great religions, and yes that is most certainly flamebait.
If I might make a suggestion, it might be best to take a step back, and instead ask "What do we know about the religious beliefs of Mohammed's parents?" You can then throw in the anecdotal story you heard, but I'd still make an explicit point of the fact that you are skeptical (which clearly you are, or you wouldn't be asking).
Now, if phrased in this way, one could make an argument that the proper stack for it is the Islam stack. I'd still vote for accepting it here (just on the basis that the religious stacks tend to be populated by folks who are, erm, a bit less "science encumbered" than most of the other stacks).
We can accept, and have accepted, history-based questions about religious figures. However, if they aren't very carefully and neutrally worded, someone is going to feel personally affronted, and flamage will ensue. So please be unusually considerate when wording such questions.