Mishneh Torah — Transmission of the Oral Law מסירת תורה שבעל פה, Chapter 26

וְכֵן יִתְבָּאַר מֵהֶם הַמִּנְהָגוֹת וְהַתַּקָּנוֹת שֶׁהִתְקִינוּ אוֹ שֶׁנָּהֲגוּ בְּכָל דּוֹר וָדוֹר, כְּמוֹ שֶׁרָאוּ בֵּית דִּין שֶׁלְּאוֹתוֹ הַדּוֹר, לְפִי שֶׁאָסוּר לָסוּר מֵהֶם, שֶׁנֶּאֱמָר "לֹא תָסוּר, מִכָּל הַדָּבָר אֲשֶׁר־יַגִּידוּ לְךָ – יָמִין וּשְׂמֹאל" (ראה דברים יז, יא).

Similarly, it includes the customs and ordinances that were ordained or practiced in each generation according to [the judgment of] the governing court of that generation. and the ordinances of Ushia, which require a father to support his children. The Rambam also discusses these three categories of Rabbinic decrees in Hilchot Mamrim, Chapters 1 and 2. It is forbidden to deviate from [these decisions], as [implied by Deuteronomy 17:11]: "Do not deviate from the instructions that they will give you, left or right."