Midrash for Pirkei Avot 1:1
משֶׁה קִבֵּל תּוֹרָה מִסִּינַי, וּמְסָרָהּ לִיהוֹשֻׁעַ, וִיהוֹשֻׁעַ לִזְקֵנִים, וּזְקֵנִים לִנְבִיאִים, וּנְבִיאִים מְסָרוּהָ לְאַנְשֵׁי כְנֶסֶת הַגְּדוֹלָה. הֵם אָמְרוּ שְׁלשָׁה דְבָרִים, הֱווּ מְתוּנִים בַּדִּין, וְהַעֲמִידוּ תַלְמִידִים הַרְבֵּה, וַעֲשׂוּ סְיָג לַתּוֹרָה:
Moses received Torah from Sinai. [I say that because this tractate is not founded on any explanation of a mitzvah of the mitzvoth of the Torah as are the other tractates of the Mishnah, but is entirely mussar and middoth, and the sages of the gentiles have also written books from the musings of their heart on the ways of mussar — how a man should deport himself with his neighbor — therefore, the tanna begins this tractate: "Moses received Torah from Sinai," to teach us that the middoth and mussar in this tractate were not conjured up by the sages of the Mishnah, but these, too, were stated on Sinai, (i.e., by Him who revealed Himself on Sinai)]. And He gave it to Joshua; and Joshua, to the elders, [who lived on after Joshua, until the advent of the first prophets, Eli the high-priest and Shmuel Haramati]; and the elders, to the prophets; and the prophets gave it to the men of the great assembly (anshei knesseth hagedolah). [They were 120 elders: Zerubaval, Seraya, Re'elayah, Mordecai-Bilshan, who lived in the days of Ezra, when they went up from the exile to the second Temple, among them, Chaggai, Zechariah, Malachi, and Nechemiah ben Chachalyah and their colleagues. They were called "the men of the great assembly" for having restored the "Crown" to its pristine (greatness). For Moses said (Deuteronomy 10:17): "The great, mighty, awesome, G-d." Jeremiah and Daniel came and (Daniel) did not say "great," and (Jeremiah) did not say "awesome," and they (the men of the great assembly) restored them ("great and awesome") as at first," saying "this (as stated there [Yoma 69b]) is precisely His greatness"; "this is precisely His awesomeness"; "for if not so (i.e., if He were not great and awesome), how could one nation endure against seventy nations (intent upon annihilating it, etc.)!"] They were wont to say three things. [They said many things; but they were wont to say these three things to uphold the integrity of Torah]: Be patient ("metunim") in judgment [i.e., if something comes before you for judgment, do not say: "Such a case has come before me two or three times before," but be patient; that is, "wait" ('mamtinim') before you rule upon it.] And set up many disciples, [(as opposed to the view of R. Gamliel, who says (Berachoth 28a): "Let no disciple whose inside is not like his outside enter the house of study.") We are hereby taught that Torah is taught to every man, and there is no need to "search him out," so long as he is not known to be a man of ill conduct and of ill repute. Or, we are (hereby) being taught that if one set up disciples in his youth, he should continue doing so in his old age, as it is written (Koheleth 11:6): "In the morning sow your seed, and in the evening let your hand not rest."] And make a fence for the Torah, [so that you not come to violate the issur of the Torah itself (e.g., the secondary class [shniyoth] of forbidden relations) and shvuth (rabbinically interdicted occupation) on Sabbath, as it is written (Leviticus 18:30): "And you shall keep My charge" — Make a "keeping" for My charge.]