Medieval Christian polemic framed the Koran as Jewish and heretical pastiche.

Evidence

Finally realizing that a feigned religious vocation would serve his ambitions, he claimed that he was a prophet and usurped the authority of king. Then, at the bidding of Satan, a heretical Nestorian monk named Sergius came and joined Muhammad: together, along with several Jews, they forged a new heretical doctrine. “Muhammad, schooled in this way by the finest teachers—Jews and heretics—composed his Koran. He wove together, in his barbarous fashion, nefarious scripture from the fables of the Jews and the ditties of the heretics.” — John V. Tolan, Saracens, 842