Indeed, even if Muhammad actually existed.
http://www.islamchristianforum.com/index.php?topic=4706.0And so often did the Quraish recognize Muhammad's recitations and accuse Muhammad of pirating and plagiarizing that he had to arrange for a special "revelation" to answer to those who knew that he was copying "tales of the ancients" (which in Mecca were tales of Arabian pagans):
http://www.searchtruth.com/search.php?keyword=tales+ancients&chapter=&translator=2&search=1&start=0&records_display=10&search_word=allOf them there are some who (pretend to) listen to thee; but We have thrown veils on their hearts, So they understand it not, and deafness in their ears; if they saw every one of the signs, not they will believe in them; in so much that when they come to thee, they (but) dispute with thee; the Unbelievers say: "These are nothing but tales of the ancients."
When Our Signs are rehearsed to them, they say: "We have heard this (before): if we wished, we could say (words) like these: these are nothing but tales of the ancients."
When it is said to them, "What is it that your Lord has revealed?" they say, "tales of the ancients!"
"Such things have been promised to us and to our fathers before! they are nothing but tales of the ancients!"
And they say: "tales of the ancients, which he has caused to be written: and they are dictated before him morning and evening."
"It is true we were promised this,- we and our fathers before (us): these are nothing but tales of the ancients."
But (there is one) who says to his parents, "Fie on you! Do ye hold out the promise to me that I shall be raised up, even though generations have passed before me (without rising again)?" And they two seek Allah's aid, (and rebuke the son): "Woe to thee! Have faith! for the promise of Allah is true." But he says, "This is nothing but tales of the ancients!"
When to him are rehearsed Our Signs, "tales of the ancients", he cries!
When Our Signs are rehearsed to him, he says, "tales of the ancients!" Muhammad alludes to it here too:
When We substitute one revelation for another,- and Allah knows best what He reveals (in stages),- they say, "Thou art but a forger": but most of them understand not.