Numbers 8:20-22

20And Moses, and Aaron, and all the congregation of the children of Israel, did to the Levites according unto all that the Lord commanded Moses concerning the Levites, so did the children of Israel unto them.

21And the Levites were purified, and they washed their clothes; and Aaron offered them as an offering before the Lord; and Aaron made an atonement for them to cleanse them.

22And after that went the Levites in to do their service in the tabernacle of the congregation before Aaron, and before his sons: as the Lord had commanded Moses concerning the Levites, so did they unto them.