וְקִרְע֤וּ לְבַבְכֶם֙ וְאַל־בִּגְדֵיכֶ֔ם וְשׁ֖וּבוּ אֶל־יְהוָ֣ה אֱלֹֽהֵיכֶ֑ם כִּֽי־חַנּ֤וּן וְרַחוּם֙ ה֔וּא אֶ֤רֶךְ אַפַּ֙יִם֙ וְרַב־חֶ֔סֶד וְנִחָ֖ם עַל־הָרָעָֽה׃
И раздираешь сердце твое, а не одежду твою, и обращайся к Господу, Богу твоему; Ибо Он милостив и сострадателен, многострадален и щедр на милость, и раскаяется в Нем от зла.
Mishnah Ta'anit
What is the order [of service] for fast days?They take the ark out to the open space of the city. And they put ashes on the ark and on the head of the Nasi and on the head of the head of the court (av bet. And everyone [else] puts ashes on his own head. The elder among them says in front of them words of admonition, “Brothers, it does not say of the people of Nineveh, ‘And God saw their sackcloth and their fasting,’ but, ‘And God saw their deeds, for they turned from their evil way. (Jonah 3:10)’ And in the prophets it says, ‘And rend your heart and not your garments” (Joel 2:13).
Pirkei Avot
Rabbi Shimon said: Be careful with the reading of Shema and the prayer, And when you pray, do not make your prayer something automatic, but a plea for compassion before God, for it is said: “for He is gracious and compassionate, slow to anger, abounding in kindness, and renouncing punishment” (Joel 2:13); And be not wicked in your own esteem.