You are children of your God יהוה. You shall not gash yourselves or shave the front of your heads because of the dead.
בָּנִ֣ים אַתֶּ֔ם לַיהֹוָ֖ה אֱלֹהֵיכֶ֑ם לֹ֣א תִתְגֹּֽדְד֗וּ וְלֹֽא־תָשִׂ֧ימוּ קׇרְחָ֛ה בֵּ֥ין עֵינֵיכֶ֖ם לָמֵֽת׃
For you are a people consecrated to your God יהוה: your God יהוה chose you from among all other peoples on earth to be a treasured people.
כִּ֣י עַ֤ם קָדוֹשׁ֙ אַתָּ֔ה לַיהֹוָ֖ה אֱלֹהֶ֑יךָ וּבְךָ֞ בָּחַ֣ר יְהֹוָ֗ה לִֽהְי֥וֹת לוֹ֙ לְעַ֣ם סְגֻלָּ֔ה מִכֹּל֙ הָֽעַמִּ֔ים אֲשֶׁ֖ר עַל־פְּנֵ֥י הָאֲדָמָֽה׃ {ס}
לא תתגדדו YE SHALL NOT CUT YOURSELVES — i.e. you shall not make cuttings and incisions in your flesh for the dead in the way the Amorites do (Sifrei Devarim 96:11), because you are children of the Lord and it is therefore becoming for you to be comely and not cut about and with hair torn out.
לא תתגדדו. לֹא תִתְּנוּ גְדִידָה וְשֶׂרֶט בִּבְשַׂרְכֶם עַל מֵת כְּדֶרֶךְ שֶׁהָאֱמוֹרִיִּים עוֹשִׂין, לְפִי שֶׁאַתֶּם בָּנָיו שֶׁל מָקוֹם וְאַתֶּם רְאוּיִין לִהְיוֹת נָאִים וְלֹא גְּדוּדִים וּמְקֹרָחִים:
Hosszú szugja, 3 oldal a Talmudban.
Reish Lakish said to Rabbi Yoḥanan: I should read here the verse: “You shall not cut yourselves [titgodedu]” (Deuteronomy 14:1), which is interpreted as meaning: Do not become numerous factions [agudot]. In other words, the Jewish people should be united, rather than divided into disparate groups that act in different ways. Before analyzing this issue, the Gemara asks: This verse: “You shall not cut yourselves,” is required for the matter itself, as the Merciful One is saying: Do not cut yourselves over the dead. How is the halakha concerning factions derived from this apparently straightforward verse?
אֲמַר לֵיהּ רֵישׁ לָקִישׁ לְרַבִּי יוֹחָנָן: אִיקְּרִי כָּאן ״לֹא תִתְגּוֹדְדוּ״, לֹא תֵּעָשׂוּ אֲגוּדּוֹת אֲגוּדּוֹת? הַאי ״לֹא תִתְגּוֹדְדוּ״ מִיבְּעֵי לֵיהּ לְגוּפֵיהּ, דְּאָמַר רַחֲמָנָא: לֹא תַּעֲשׂוּ חַבּוּרָה עַל מֵת!