Numbers 8:7

הזה עליהם מי חטאת SPRINKLE UPON THEM WATER OF PURIFYING — water in which the ashes of the Red Heifer are mingled; on account of those among them who might have become unclean through contact with dead persons. והעבירו תער AND LET THEM SHAVE [ALL THEIR FLESH] — I have found in the work of R. Moses the Preacher the following: Because they were made propitiatory substitutes for the firstborn who had worshipped the idol (the Golden Calf), and it (idolatry) is called (Psalms 106:28) “offerings to the dead”, and the leper is also called dead (cf. Numbers 12:12), it (Scripture) requires them to shave their body like lepers.

וְכֹֽה־תַעֲשֶׂ֤ה לָהֶם֙ לְטַֽהֲרָ֔ם הַזֵּ֥ה עֲלֵיהֶ֖ם מֵ֣י חַטָּ֑את וְהֶעֱבִ֤ירוּ תַ֙עַר֙ עַל־כׇּל־בְּשָׂרָ֔ם וְכִבְּס֥וּ בִגְדֵיהֶ֖ם וְהִטֶּהָֽרוּ׃

And thus shalt thou do to them, to cleanse them: Sprinkle water of purifying on them, and let them shave all their flesh, and let them wash their clothes, and so make themselves clean.