Numbers 8:17-19

Young's Literal Translation

¹⁷ for Mine [is] every first-born among the sons of Israel, among man and among beast; in the day of my smiting every first-born in the land of Egypt I sanctified them for Myself;

¹⁸ and I take the Levites instead of every first-born among the sons of Israel:

¹⁹ `And I give the Levites gifts to Aaron and to his sons, from the midst of the sons of Israel, to do the service of the sons of Israel in the tent of meeting, and to make atonement for the sons of Israel, and there is no plague among the sons of Israel in the sons of Israel's drawing nigh unto the sanctuary.'