Numbers 8:18-22

But I have taken the Levites instead of every firstborn among the sons of Israel.

And I have given the Levites as a gift to Aaron and to his sons from among the sons of Israel, to perform the service of the sons of Israel at the tent of meeting and to make atonement on behalf of the sons of Israel, so that there will be no affliction among the sons of Israel due to their approaching the sanctuary.”

So this is what Moses, Aaron, and all the congregation of the sons of Israel did to the Levites; according to everything that the Lord had commanded Moses regarding the Levites, so the sons of Israel did to them.

The Levites, too, purified themselves from sin and washed their clothes; and Aaron presented them as a wave offering before the Lord. Aaron also made atonement for them to cleanse them.

Then after that the Levites went in to perform their service in the tent of meeting before Aaron and his sons; just as the Lord had commanded Moses concerning the Levites, so they did to them. Retirement for the Levites