"Did not Moses give you the law"
"If a man on the sabbath day receive circumcision, that the law of Moses should not be broken; are ye angry at me, because I have made a man every whit whole on the sabbath day?"
"These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me."
This last passage especially divides the OT into the Pentateuch (the Law), the prophets and the psalms. The Law doesn't mean the 10 commandments, and doesn't mean the extended laws of the Pentateuch but stands for those 5 books. I am sure Br Max can expand with a great deal more insight and information.
Just as no Jew would have doubted that Moses wrote the Pentateuch, so there was no doubt among the Lord and his Apostles.