Numbers 8:14-16

“Thus you shall separate the Levites from among the sons of Israel, and the Levites shall be Mine. Then 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; for they are wholly given to Me from among the sons of Israel. I have taken them for Myself instead of every first issue of the womb, the firstborn of all the sons of Israel.