וַיֹּ֨אמֶר יְהוָ֤ה אֶל־מֹשֶׁה֙ לֵ֣ךְ אֶל־הָעָ֔ם וְקִדַּשְׁתָּ֥ם הַיּ֖וֹם וּמָחָ֑ר וְכִבְּס֖וּ שִׂמְלֹתָֽם׃
Lalu Yang Mahakekal berfirman kepada Musa: “Pergilah kepada umat itu dan persiapkanlah mereka hari ini dan besok; hendaklah mereka mencuci pakaian mereka,
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.