Mishnah 8 - Chapter 3 - Pirkei Avot

Rabbi doseta berabbi yanna mishum rabbi me'ir omer. Kol hashocheach davar echad mimmishenato, ma'aleh alav hakkatuv ke'illu mitchayev benafsho, shenne'emar (dvrm d), rak hishamer lecha ushemor nafshecha me'od pen tishkach et haddevarim asher ra'u eineicha. Yachol afillu takefah alav mishenato, talmud lomar (shm,) ufen yasuru millevavecha kol yemei chayeicha, ha eino mitchayev benafsho ad sheyeshev visirem millibbo.

Rabbi Dostai ben Rabbi Yannai said in the name of Rabbi Meir: whoever forgets one word of his study, scripture accounts it to him as if he were mortally guilty, as it is said, “But take utmost care and watch yourselves scrupulously, so that you do not forget the things that you saw with your own eyes” (Deuteronomy 4:9). One could [have inferred that this is the case] even when his study proved [too] hard for him, therefore scripture says, “that they do not fade from your mind as long as you live” (ibid.). Thus, he is not mortally guilty unless he deliberately removes them from his heart.

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.