וַיֹּ֨אמֶר יְהוָ֤ה אֶל־מֹשֶׁה֙ לֵ֣ךְ אֶל־הָעָ֔ם וְקִדַּשְׁתָּ֥ם הַיּ֖וֹם וּמָחָ֑ר וְכִבְּס֖וּ שִׂמְלֹתָֽם׃
И сказал Господь Моисею: 'Иди к народу и освяти его сегодня и завтра и позволь им постирать свои одежды,
Mishneh Torah, Forbidden Intercourse
Immersion was in the desert before receiving the Torah, as is said, "Sanctify yourselves today and tomorrow; wash your clothes." The offering, as is said, "Send the young ones of Israel to bring elevation offerings" - and everyone in Israel brought them.