Numbers 8:20-22

Thus Moses and Aaron and all the congregation of the children of Israel did to the Levites; according to all that the LORD commanded Moses concerning the Levites, so the children of Israel did to them. And the Levites purified themselves and washed their clothes; then Aaron presented them like a wave offering before the LORD, and Aaron made atonement for them to cleanse them. After that the Levites went in to do their work in the tabernacle of meeting before Aaron and his sons; as the LORD commanded Moses concerning the Levites, so they did to them.