Numbers 8:21 in the King James Version
And the Levites were purified, and they washed their clothes; and Aaron offered them as an offering before the LORD; and Aaron made an atonement for them to cleanse them.
Context
20 Moses, and Aaron, and all the congregation of the children of Israel did so to the Levites. According to all that the LORD commanded Moses concerning the Levites, so the children of Israel did to them.
21 The Levites purified themselves from sin, and they washed their clothes; and Aaron offered them for a wave offering before the LORD and Aaron made atonement for them to cleanse them.
22 After that, the Levites went in to do their service in the Tent of Meeting before Aaron and before his sons: as the LORD had commanded Moses concerning the Levites, so they did to them.