Musar for Bava Metzia 65:3

Repentance by following what is written — what is meant by that ? If he had intercourse with his wife during her unclean period, for which transgression the penalty is that he be cut off from his people, or if he committed any of the sins for which he must be sentenced to death by a court of law, or to be lashed, let him accept it upon himself to suffer pain and let him inflict pain upon himself, as it is written in the Torah. Now, the Rokeah ^** Eleazar of Worms (died 1238). has written how one may inflict pain upon himself for various sins. And we also find in the Talmud that they were accustomed to fast even for a very minor sin, even for mere talk. As in the case of the Sage who said, "1 am ashamed of your words, oh House of Shammai." And because of this remark he sat fasting until his teeth became black (Hagigah 22b). And there is also a story concerning Rabbi Hisda who said to Rabbi Huna on the question of respect due to another, "What of a disciple whom his teacher needs (because the disciple possesses traditions received from other scholars and which the teacher does not know), does this disciple have to stand in honor of the teacher?" Rabbi Huna retorted to Rabbi Hisda, "I do not need you." And for this apparent slight each one sat fasting many times (Baba Mezi'a 33a). And so have we found, concerning David, that the Divine Presence and the Holy Spirit departed from him for twenty-two years and every day he would shed tears and eat his morsel of food dipped in ashes. As it is said (Ps. 102:10), "For I have eaten ashes like bread, and mingled my drink with weeping." David said to God, "Master of the Universe, receive me as completely repentant before Thee, in order that I may purify the wicked in the world, as it is said (Ps. 51:15), "Then will I teach transgressors Thy ways, and sinners shall return unto Thee" (Tana Debé Eliahu, chap. 2).