Numbers 8:20-22

20Thus Moses and Aaron and all the congregation of the children of Israel did [these things] to the Levites; in accordance with all that the Lord had commanded Moses concerning the Levites, so the Israelites did to them.

21The Levites, too, purified themselves from sin and they washed their clothes; and Aaron presented them as a wave offering before the Lord, and Aaron made atonement for them to cleanse them.

22Then after that the Levites went in to perform their service in the Tent of Meeting before Aaron and his sons; just as the Lord had commanded Moses concerning the Levites, so they did to them.