Leviticus 10:6

ראשיכם] אל תפרעו] means, let not your hair grow long (cf. Numbers 6:5) (Sifra, Shemini, Mechilta d'Miluim 2 40). From this (from the fact that Scripture forbade these particular mourners to let their hair grow long) it follows that an אבל (one mourning the death of a near relative) is forbidden to cut his hair. Moses's words therefore signified: Ordinarily an אבל may not cut his hair but ye, disturb ye not the joy of the Omnipresent God by displaying signs of mourning (Moed Katan 14b). ולא תמתו LEST YE DIE — Consequently if ye do this ye shall die (Sifra, Shemini, Mechilta d'Miluim 2 41). ואחיכם כי בית ישראל BUT (LET] YOUR BRETHREN, THE WHOLE OF ISRAEL [WEEP] — From this it follows that for the miseries of the learned the duty devolves upon everyone (the entire community) to mourn (cf. Moed Katan 28b.)

וַיֹּ֣אמֶר מֹשֶׁ֣ה אֶֽל־אַהֲרֹ֡ן וּלְאֶלְעָזָר֩ וּלְאִֽיתָמָ֨ר ׀ בָּנָ֜יו רָֽאשֵׁיכֶ֥ם אַל־תִּפְרָ֣עוּ ׀ וּבִגְדֵיכֶ֤ם לֹֽא־תִפְרֹ֙מוּ֙ וְלֹ֣א תָמֻ֔תוּ וְעַ֥ל כׇּל־הָעֵדָ֖ה יִקְצֹ֑ף וַאֲחֵיכֶם֙ כׇּל־בֵּ֣ית יִשְׂרָאֵ֔ל יִבְכּוּ֙ אֶת־הַשְּׂרֵפָ֔ה אֲשֶׁ֖ר שָׂרַ֥ף יְהֹוָֽה׃

And Moshe said to Aharon, and to El῾azar and to Itamar, his sons, Let the hair of your heads not grow long, neither rend your clothes; lest you die, and lest anger come upon all the people: but let your brethren, the whole house of Yisra᾽el bewail the burning which the Lord has kindled.