Numbers 8:20-22

20Thus did Moses and Aaron and all the congregation of the sons of Israel to the Levites; according to all that the Lord had commanded Moses concerning the Levites, so the sons of Israel did to them. 211The Levites, too, purified themselves from sin and washed their clothes; and Aaron presented them as a wave offering before the Lord. Aaron also made atonement for them to cleanse them. 22Then after that the Levites went in to perform their service in the tent of meeting before Aaron and before his sons; just as the Lord had commanded Moses concerning the Levites, so they did to them.