Numbers 8:17-19

17“For every firstborn among Bnei-Yisrael is Mine, whether human or animal. On the day I struck down the firstborn of the land of Egypt, I sanctified them for Myself.

18So I am taking the Levites in place of the firstborn of Bnei-Yisrael,

19and I am giving the Levites as a gift to Aaron and his sons from among Bnei-Yisrael, to do the work on behalf of Bnei-Yisrael in the Tent of Meeting, and to make atonement for them—so that there would be no plague among them for coming too close to the Sanctuary.”