Moses spent several periods of 40 days in supernatural communion with God, and then spent 40 years teaching the Children of Israel what he had learned. Only as his death was near did Moses make sure to leave scrolls, the ‘Five Books’.
The teachings came before The Book. Those teachers who trace their teachers most directly back to Moses are the best source authority.
The Mishna and Talmud record sayings, hypothetical questions, and disagreements between ancient master students of the Torah of Moses.
Word of mouth is notoriously unreliable.
That is no authority.