Numbers 8:17-19

17For 1every firstborn among the sons of Israel is Mine, among the men and among the animals; on the day that I struck down 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. 192I 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 4plague among the sons of Israel by 5their coming near to the sanctuary.”