Numbers 8:17-19

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.”