Numbers 8:13-18

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 the Levites shall be Mine. ¹⁵Then after that the Levites may go in to serve the tent of meeting. But you shall cleanse them and present them as a wave offering; ¹⁶for they are exclusively given to Me from among the sons of Israel. I have taken them for Myself instead of the firstborn of every womb, the firstborn of all the sons of Israel. ¹⁷For every 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.