Numbers 8:20-22

20So Moses and Aaron and the entire community of the Israelites did this with the Levites. According to all that the Lord commanded Moses concerning the Levites, this is what the Israelites did with them.

21The Levites purified themselves and washed their clothing; then Aaron presented them like a wave offering before the Lord, and Aaron made atonement for them to purify them.

22After this, the Levites went in to do their work in the tent of meeting before Aaron and before his sons. As the Lord had commanded Moses concerning the Levites, so they did.