The thing is his Arab audience had no idea what was in the Christian and Jewish sources so Muhammad could easily make stuff up without being held into account. He could say he was prophesied in their scriptures, he could say his revelation confirm theirs and he could recite Jewish tradition/embelishment as part and parcel of the original Torah stories. It was when they encountered some Jews who laughed his out of town and called his revelation fables and tales of the ancients that Muhammad framed the Jews for being evil by concealing the fact that he is prophesied in their scriptures and it lay the groundwork for the latter revelations calling the Jews cursed and the command to fight and kill them.