Numbers 8:19-21

Amplified Bible

¹⁹ I have given the Levites as a gift to Aaron and to his sons from among the Israelites, to perform the service of the Israelites at the Tent of Meeting (tabernacle) and to make atonement for them, so that there will be no plague among the Israelites if they should approach the sanctuary.”

²⁰ Thus 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. ²¹ The 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.