Numbers 8:21-22

King James Version

²¹ And 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.

²² And 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.