Mishnà su Deuteronomio 4:78

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.

Rabbi Ishmael his son said: He who learns in order to teach, it is granted to him to study and to teach; But he who learns in order to practice, it is granted to him to learn and to teach and to practice. Rabbi Zadok said: do not make them a crown for self-exaltation, nor a spade with which to dig. So too Hillel used to say, “And he that puts the crown to his own use shall perish.” Thus you have learned, anyone who derives worldly benefit from the words of the Torah, removes his life from the world.