Numbers 8:20-22

Moses, Aaron, and the entire community of the People of Israel carried out these procedures with the Levites, just as GOD had commanded Moses. The Levites purified themselves and scrubbed their clothes. Then Aaron presented them as a Wave-Offering before GOD and made atonement for them to purify them. Only then did the Levites go to work at the Tent of Meeting. Aaron and his sons supervised them following the directions GOD had given.