Numbers 8:20-22

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 ¹  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.

[8:21]  ¹ tn The verb is the Hitpael of חָטָּא (khatta’). In this stem the meaning of the root “to sin” is likely to be connected to the noun “sin/purification” offering in a denominative sense, although some would take it as a privative usage, “to remove sin.” The idea is clear enough: They performed all the ritual in order to purify themselves ceremonially.