Pirkei Avot §avot_03_08: Rabbi Dostai ben Rabbi Yannai said in the name of Rabbi Meir: whoever forgets...

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.