Numbers 8:6-11, 21

“Take the Levites from among all the Israelites and make them ceremonially clean. To purify them, do this: Sprinkle the water of cleansing on them; then have them shave their whole bodies and wash their clothes. And so they will purify themselves. Have them take a young bull with its grain offering of the finest flour mixed with olive oil; then you are to take a second young bull for a sin offering. Bring the Levites to the front of the tent of meeting and assemble the whole Israelite community. You are to bring the Levites before the LORD, and the Israelites are to lay their hands on them. Aaron is to present the Levites before the LORD as a wave offering from the Israelites, so that they may be ready to do the work of the LORD.

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.