Numbers 8:20-22

Moses, Aaron and the whole Israelite community did with the Levites just as the LORD commanded Moses. The Levites purified themselves and washed their clothes. Then Aaron presented them as a wave offering before the LORD and made atonement for them to purify them. After that, the Levites came to do their work at the tent of meeting under the supervision of Aaron and his sons. They did with the Levites just as the LORD commanded Moses.