Mishneh Torah — Transmission of the Oral Law מסירת תורה שבעל פה, Chapter 32

וְכָל בֵּית דִּין שֶׁעָמַד אַחַר הַתַּלְמוּד בְּכָל מְדִינָה וּמְדִינָה וְגָזַר אוֹ הִתְקִין אוֹ הִנְהִיג לִבְנֵי מְדִינָתוֹ, אוֹ לִבְנֵי מְדִינוֹת – לֹא פָשְׁטוּ מַעֲשָׂיו בְּכָל יִשְׂרָאֵל: מִפְּנֵי רֹחַק מוֹשְׁבוֹתֵיהֶם, וְשִׁבּוּשׁ הַדְּרָכִים; וֶהֱיוֹת בֵּית דִּין שֶׁלְּאוֹתָהּ הַמְּדִינָה יְחִידִים, וּבֵית דִּין הַגָּדוֹל שֶׁלְּשִׁבְעִים בָּטַל מִכַּמָּה שָׁנִים קֹדֶם חִבּוּר הַתַּלְמוּד.

Every court that was established after the conclusion of the Talmud, regardless of the country in which it was established, issued decrees, enacted ordinances, and established customs for the people of that country - or those of several countries. These practices, however, were not accepted throughout the Jewish people, because of the distance between [their different] settlements and the disruption of communication [between them]. Since each of these courts were considered to be individuals - and the High Court of 71 judges had been defunct for many years before the composition of the Talmud -