Musar על אבות 4:5

But if he occupies himself with the study of the Torah and teaching it to his pupils, happy is he, for this is the best and most righteous occupation. For if he should err they will warn him, and if he should forget they will remind him, and if he should sleep they will arouse him, and if he should be wicked they will consider him righteous in spite of himself. As for him who occupies himself with the study of the Torah and repents only a little and so clings to his wickedness, know that if he were to engage in any other occupation, his wickedness would be double and his evil twice as much. Therefore, one should occupy oneself with the study of the Torah and be very careful with that precious treasure which the Creator gave to him. Let him not make of it an instrument to attain his desires and his pleasures, as our Sages, of blessed memory, said (Ethics of the Fathers Chapter 4), “Do not make them a crown with which to vaunt yourself, nor a spade with which to dig.” When a man teaches someone else or admonishes his pupils, let him not forget to punish his own soul the while, and to teach it, for it is not proper for a man to instruct someone else in what he himself does not do, and to teach him what he does not believe. For anything that comes out of the heart will enter into the heart, while anything that comes out of the mouth only will not go beyond the ear. A wise man said, “Whenever a man admonishes me and I want to know if that admonishment comes from the heart, I look to see if his instruction has entered my heart and aroused my soul to pay attention to his words. If so, I recognize that he is speaking with all his heart and all his might. But if my soul does not pay attention to his words and has not aroused itself to cling to them, then I know that his rebuke is nothing but the word of his lips.”