Mishneh Torah — Transmission of the Oral Law מסירת תורה שבעל פה, Chapter 30

וְאַחַר בֵּית דִּינוֹ שֶׁלְּרָב אַשֵׁי, שֶׁחִבַּר הַתַּלְמוּד בִּימֵי בְּנוֹ וּגְמָרוֹ, נִתְפַּזְּרוּ יִשְׂרָאֵל בְּכָל הָאֲרָצוֹת פִּזּוּר יָתֵר, וְהִגִּיעוּ לַקְּצָווֹת וְלָאִיִּים הָרְחוֹקִים; וְרָבְתָה קְטָטָה בָּעוֹלָם, וְנִשְׁתַּבְּשׁוּ הַדְּרָכִים בִּגְיָסוֹת. וְנִתְמַעַט תַּלְמוּד תּוֹרָה, וְלֹא נִתְכַּנְּסוּ יִשְׂרָאֵל לִלְמֹד בִּישִׁיבוֹתֵיהֶם אֲלָפִים וּרְבָבוֹת כְּמוֹ שֶׁהָיוּ מִקֹּדֶם.

After the court of Rav Ashi composed the Talmud and completed it in the time of his son, the Jewish people became further dispersed throughout all the lands, reaching the distant extremes and the far removed islands. Strife sprung up throughout the world, and the paths of travel became endangered by troops. Torah study decreased and the Jews ceased entering their yeshivot in the thousands and myriads, as was customary previously.