Numbers 8:19-22

8:19 I have given the Levites as a gift to Aaron and his sons from among the Israelites, to do the work for the Israelites in the tent of meeting, and to make atonement for the Israelites, so there will be no plague among the Israelites when the Israelites come near the sanctuary.”

8:20 So 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. 8:21 The 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. 8:22 After 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.