Numbers 8:14-16

Thus you shall separate the Levites from among the children of Israel, and the Levites shall be Mine. After that the Levites shall go in to service the tabernacle of meeting. So you shall cleanse them and offer them like a wave offering. For they are wholly given to Me from among the children of Israel; I have taken them for Myself instead of all who open the womb, the firstborn of all the children of Israel.