Numbers 8:13-19

13Then have the Levites stand in front of Aaron and his sons, and raise your hands and present them as a special offering to the Lord.

14In this way, you will set the Levites apart from the rest of the people of Israel, and the Levites will belong to me.

15After this, they may go into the Tabernacle to do their work, because you have purified them and presented them as a special offering.

16“Of all the people of Israel, the Levites are reserved for me. I have claimed them for myself in place of all the firstborn sons of the Israelites; I have taken the Levites as their substitutes.

17For all the firstborn males among the people of Israel are mine, both of people and of animals. I set them apart for myself on the day I struck down all the firstborn sons of the Egyptians.

18Yes, I have claimed the Levites in place of all the firstborn sons of Israel.

19And of all the Israelites, I have assigned the Levites to Aaron and his sons. They will serve in the Tabernacle on behalf of the Israelites and make sacrifices to purify the people so no plague will strike them when they approach the sanctuary.”