Numbers 8:18-20

Amplified Bible

¹⁸ And I have taken the Levites instead of every firstborn among the Israelites. ¹⁹ I have given the Levites as a gift to Aaron and to his sons from among the Israelites, to perform the service of the Israelites at the Tent of Meeting (tabernacle) and to make atonement for them, so that there will be no plague among the Israelites if they should approach the sanctuary.”

²⁰ Thus Moses and Aaron and all the congregation of the children of Israel did [these things] to the Levites; in accordance with all that the Lord had commanded Moses concerning the Levites, so the Israelites did to them.