Numbers 8:17-19

New Revised Standard Version, Anglicised Catholic Edition

¹⁷ For all the firstborn among the Israelites are mine, both human and animal. On the day that I struck down all the firstborn in the land of Egypt I consecrated them for myself, ¹⁸ but I have taken the Levites in place of all the firstborn among the Israelites. ¹⁹ Moreover, I have given the Levites as a gift to Aaron and his sons from among the Israelites, to do the service for the Israelites at the tent of meeting, and to make atonement for the Israelites, in order that there may be no plague among the Israelites for coming too close to the sanctuary.