Mishneh Torah — Transmission of the Oral Law מסירת תורה שבעל פה, Chapter 13

וְכֵן כָּל אֶחָד וְאֶחָד כּוֹתֵב לְעַצְמוֹ כְּפִי כּוֹחוֹ, מִבֵּאוּר הַתּוֹרָה וּמֵהִלְכּוֹתֶיהָ כְּמוֹ שֶׁשָּׁמַע, וּמִדְּבָרִים שֶׁנִּתְחַדְּשׁוּ בְּכָל דּוֹר וָדוֹר, בְּדִינִים שֶׁלֹּא לְמָדוּם מִפִּי הַשְּׁמוּעָה אֵלָא בְּמִדָּה מִשְּׁלוֹשׁ עֶשְׂרֵה מִדּוֹת וְהִסְכִּימוּ עֲלֵיהֶן בֵּית דִּין הַגָּדוֹל. וְכֵן הָיָה הַדָּבָר תָּמִיד, עַד רַבֵּנוּ הַקָּדוֹשׁ.

Similarly, according to his own potential, each individual would write notes for himself of what he heard regarding the explanation of the Torah, its laws, and the new concepts that were deduced in each generation concerning laws that were not communicated by the oral tradition, but rather deduced using one of the thirteen principles of Biblical exegesis and accepted by the high court. This situation continued until [the age of] Rabbenu Hakadosh.