Mishneh Torah — Transmission of the Oral Law מסירת תורה שבעל פה, Chapter 27

וְכֵן מִשְׁפָּטִים וְדִינִין פִּלְאִיִּים שֶׁלֹּא קִבְּלוּ אוֹתָן מִמֹּשֶׁה, וְדָנוּ בָּהֶן בֵּית דִּין הַגָּדוֹל שֶׁלְּאוֹתוֹ הַדּוֹר בַּמִּדּוֹת שֶׁהַתּוֹרָה נִדְרֶשֶׁת בָּהֶן, וּפָסְקוּ אוֹתָן הַזְּקֵנִים, וְגָמְרוּ שֶׁהַדִּין כָּךְ הוּא. הַכֹּל חִבַּר רָב אַשֵׁי בַּתַּלְמוּד, מִיְּמוֹת מֹשֶׁה וְעַד יָמָיו.

It also includes marvelous judgments and laws which were not received from Moses, but rather were derived by the courts of the [later] generations based on the principles of Biblical exegesis. The elders of those generations made these decisions and concluded that this was the law. Rav Ashi included in the Talmud this entire [body of knowledge, stemming] from the era of Moses, our teacher, until his [own] era.