Numbers 8:17-19

International Standard Version

¹⁷ since every firstborn of Israel belongs to me, from human beings to livestock. On the same day that I destroyed all the firstborn in the land of Egypt, I consecrated them to myself, ¹⁸ taking the descendants of Levi instead of every firstborn of the Israelis. ¹⁹ I’ve set the descendants of Levi apart from the Israelis so that Aaron and his sons would work in service at the appointed place of meeting, making atonement on behalf of the Israelis so that there won’t be a plague among the Israelis whenever they approach the sanctuary.”