Numbers 8:6-11

8:6 “Take the Levites from among the Israelites and purify them. 8:7 And do this to them to purify them: Sprinkle water of purification on them; then have them shave all their body and wash their clothes, and so purify themselves. 8:8 Then they are to take a young bull with its grain offering of fine flour mixed with olive oil; and you are to take a second young bull for a purification offering. 8:9 You are to bring the Levites before the tent of meeting and assemble the entire community of the Israelites. 8:10 Then you are to bring the Levites before the Lord, and the Israelites are to lay their hands on the Levites; 8:11 and Aaron is to offer the Levites before the Lord as a wave offering from the Israelites, that they may do the work of the Lord.

Numbers 8:21

8:21 The Levites purified themselves and washed their clothing; then Aaron presented them like a wave offering before the Lord, and Aaron made atonement for them to purify them.