Numbers 8:20-22

20Thus did Moses, and Aaron, and all the congregation of the children of Israel, unto the Levites: according unto all that Jehovah commanded Moses touching the Levites, so did the children of Israel unto them. 21And the Levites purified themselves from sin, and they washed their clothes: and Aaron offered them for a wave-offering before Jehovah; and Aaron made atonement for them to cleanse them. 22And after that went the Levites in to do their service in the tent of meeting before Aaron, and before his sons: as Jehovah had commanded Moses concerning the Levites, so did they unto them.