Rav Ovadia of Bartenura
“Moshe received the Torah at Sinai” (Avot 1:1)
Unlike all the other masechtot, Avot is not based on any one of the mitzvot of the Torah. It consists only of counsel in regard to self-improvement and good character.
Wise individuals of other nations also compiled ethical works, which they developed with their own intellect, on how people should relate to one another. One might think that Avot is also a creation of man.
The Sages therefore begin this Tractate of Avot with “Moshe received the Torah at Sinai” to tell us that this masechta in not man-made but was relayed to Moshe at Sinai.