Numbers 8:20-22

Thus did Moses and Aaron and all the congregation of the people of Israel to the Levites. According to all that the LORD commanded Moses concerning the Levites, the people of Israel did to them. And the Levites purified themselves from sin and washed their clothes, and Aaron offered them as a wave offering before the LORD, and Aaron made atonement for them to cleanse them. And after that the Levites went in to do their service in the tent of meeting before Aaron and his sons; as the LORD had commanded Moses concerning the Levites, so they did to them.