Lol, awesome thread.
Here is what might happen: You would spend years inventing a time machine, then you would spend more years learning Classical Arabic, then you become an expert in poisons, then you would study the natural landmarks around Mecca and Medina so that you could locate the place when you got back in time.
Then when you finally go back to the Hejaz in the 620s AD, you start looking for Mecca and the Zam Zam Well but they are simply not there, just an empty desert and you start asking the local tribes where you can find Muhammad the Prophet and they have no idea what you're talking about. Then you realize that you were duped into believing the same silly stories as Muslims. It becomes obvious to you that the Muhammad character emerged out of legends in the subsequent century, or if he did exist, he existed in some other corner of the desert in some other decade. All of your carefully laid out plans of assassinating Muhammad were wasted because you believed too literally in the traditional narrative. You're mission is an epic failure!
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