Numbers 8:13-19

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

14“So you shall single out 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 3exclusively given to Me from among the sons of Israel. I have taken them for Myself 4instead of the firstborn of every womb, the firstborn of all the sons of Israel. 17For 5every 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 6I 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 8affliction among the sons of Israel due to 9their approaching the sanctuary.”