Numbers 8:16-18 New International Version - UK (NIVUK)
16 They are the Israelites who are to be given wholly to me. I have taken them as my own in place of the firstborn, the first male offspring from every Israelite woman. 17 Every firstborn male in Israel, whether human or animal, is mine. When I struck down all the firstborn in Egypt, I set them apart for myself. 18 And I have taken the Levites in place of all the firstborn sons in Israel.