Numbers 8:20-22

20And Moses and Aaron, and all the assembly of the children of Israel, did to the Levites according to all that Jehovah had commanded Moses concerning the Levites: so did the children of Israel to them.

21And the Levites purified themselves from sin, and they washed their garments; and Aaron offered them as a wave-offering before Jehovah; and Aaron made atonement for them to cleanse them.

22And afterwards the Levites came in to perform their service in the tent of meeting before Aaron, and before his sons; as Jehovah had commanded Moses concerning the Levites, so did they to them.