Numbers 8:17-19

Revised Standard Version

¹⁷ For all the first-born among the people of Israel are mine, both of man and of beast; on the day that I slew all the first-born in the land of Egypt I consecrated them for myself, ¹⁸ and I have taken the Levites instead of all the first-born among the people of Israel. ¹⁹ And I have given the Levites as a gift to Aaron and his sons from among the people of Israel, to do the service for the people of Israel at the tent of meeting, and to make atonement for the people of Israel, that there may be no plague among the people of Israel in case the people of Israel should come near the sanctuary.”