Numbers 8:17-19

New Catholic Bible

¹⁷ All the firstborn of the people of Israel belong to me, both human and animal. I sanctified them to myself on the day that I killed every firstborn in the land of Egypt. ¹⁸ But I have taken the Levites as a substitute for all the firstborn of the people of Israel. ¹⁹ I have given the Levites as a gift to Aaron and his sons from among the people of Israel, to perform the work of the people of Israel in the tent of meeting and to make atonement for the people of Israel, so that no plague may come upon the people of Israel when the people of Israel approach near the sanctuary.”