Musar for Pirkei Avot 3:8
רַבִּי דּוֹסְתַּאי בְּרַבִּי יַנַּאי מִשּׁוּם רַבִּי מֵאִיר אוֹמֵר, כָּל הַשּׁוֹכֵחַ דָּבָר אֶחָד מִמִּשְׁנָתוֹ, מַעֲלֶה עָלָיו הַכָּתוּב כְּאִלּוּ מִתְחַיֵּב בְּנַפְשׁוֹ, שֶׁנֶּאֱמַר (דברים ד) רַק הִשָּׁמֶר לְךָ וּשְׁמֹר נַפְשְׁךָ מְאֹד פֶּן תִּשְׁכַּח אֶת הַדְּבָרִים אֲשֶׁר רָאוּ עֵינֶיךָ. יָכוֹל אֲפִלּוּ תָקְפָה עָלָיו מִשְׁנָתוֹ, תַּלְמוּד לוֹמַר (שם) וּפֶן יָסוּרוּ מִלְּבָבְךָ כֹּל יְמֵי חַיֶּיךָ, הָא אֵינוֹ מִתְחַיֵּב בְּנַפְשׁוֹ עַד שֶׁיֵּשֵׁב וִיסִירֵם מִלִּבּוֹ:
R. Dostai the son of R. Yannai says in the name of R. Meir: If one forgets even one thing of his learning [because he did not review it], Scripture reckons it to him as if he is liable for his soul [for because he forgets it, he comes to permit what is forbidden and to create a stumbling-block. And his unwillingness is reckoned deliberate. Or, he is liable for his soul because that learning was his protection, and now that he has forgotten it, he is no longer protected], it being written (Deuteronomy 4:9): "Only take heed to yourself and heed your soul exceedingly, lest you forget the things which your eyes saw." I might think this were so even if his learning "overcame" him [i.e., even if it was difficult for him and because of its difficulty he forgot it]; it is, therefore, written (Ibid.): "and lest they depart from your heart all the days of your life" — He is not liable for his soul unless he sits and (deliberately) removes them from his heart.