Musar על מועד קטן 9:21

These works will also make clear the rules which the Sages and Prophets of every generation decreed in order to make a fence about the Torah, as they heard explicitly from the mouth of Moses, as it is said, "Therefore shall ye keep my charge" (Lev. 18:30), that is to say, make a guard for the laws which were to be guarded (Moed Katan 5a). One can also obtain from these works a clear interpretation of the customs, and the regulations that our Sages enacted in every generation according to the way that the Bet Din of that generation understood the matter, for it is forbidden to turn away from the words of the Sages, as it is said, "Thou shalt not turn aside from the sentence which they shall declare unto thee, to the right hand, nor to the left" (Deut. 17:11). And similarly included were all those judgments and excellent laws which our Sages did not receive from Moses, our teacher, but which the Bet Din of that generation decided according to the rules by which the Torah is interpreted, and also the decisions which the Elders made and concluded that the law was thus. Everything was included in the Gemara by Rav Ashi, from the days of Moses until his own day.