Numbers 8:17-19

17For 1every firstborn among the sons of Israel is Mine, among the people and among the animals; on the day that I fatally struck all the firstborn in the land of Egypt, I sanctified them for Myself. 18But I have taken the Levites instead of every firstborn among the sons of Israel. 19And 2I have given the Levites as 3a gift to Aaron and to his sons from among the sons of Israel, to perform the service of the sons of Israel at the tent of meeting and to make atonement on behalf of the sons of Israel, so that there will be no 4affliction among the sons of Israel due to 5their approaching the sanctuary.”