Numbers 8:7

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.

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.