Numbers 8:16-18

Young's Literal Translation

¹⁶ `For they are certainly given to Me out of the midst of the sons of Israel, instead of him who openeth any womb -- the first-born of all -- from the sons of Israel I have taken them to Myself;

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: