Numbers 8:13-19

13You shall have the Levites stand before Aaron and before his sons so as to present them as a wave offering to the Lord.

14“Thus you shall separate the Levites from among the sons of Israel, and 1the Levites shall be Mine. 15Then after that the Levites may go in to serve the tent of meeting. But you shall cleanse them and 2present them as a wave offering; 16for they are 3wholly given to Me from among the sons of Israel. I have taken them for Myself 4instead of every first issue of the womb, the firstborn of all the sons of Israel. 17For 5every 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. 196I have given the Levites as 7a 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 8plague among the sons of Israel by 9their coming near to the sanctuary.”