Numbers 8:16-18 New English Translation (NET Bible)
16 For they are entirely given to me from among the Israelites. I have taken them for myself instead of all who open the womb, the firstborn sons of all the Israelites. 17 For all the firstborn males among the Israelites are mine, both humans and animals; when I destroyed all the firstborn in the land of Egypt I set them apart for myself. 18 So I have taken the Levites instead of all the firstborn sons among the Israelites.