Numbers 8:6-11, 21

“Take the Levites from among the people of Israel and cleanse them. Thus you shall do to them to cleanse them: sprinkle the water of purification upon them, and let them go with a razor over all their body, and wash their clothes and cleanse themselves. Then let them take a bull from the herd and its grain offering of fine flour mixed with oil, and you shall take another bull from the herd for a sin offering. And you shall bring the Levites before the tent of meeting and assemble the whole congregation of the people of Israel. When you bring the Levites before the LORD, the people of Israel shall lay their hands on the Levites, and Aaron shall offer the Levites before the LORD as a wave offering from the people of Israel, that they may do the service of the LORD.

And the Levites purified themselves from sin and washed their clothes, and Aaron offered them as a wave offering before the LORD, and Aaron made atonement for them to cleanse them.