Mischna zu Dewarim 4:14

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.