Numbers 8:20-22

20So Moses, Aaron, and the whole congregation of Israel did with the Levites everything that the LORD had commanded Moses they should do. 21The Levites purified themselves and washed their clothes, and Aaron presented them as a wave offering before the LORD. Aaron also made atonement for them to cleanse them. 22After that, the Levites came to perform their service at the Tent of Meeting in the presence of Aaron and his sons. Thus they did with the Levites just as the LORD had commanded Moses.