Numbers 8:14-16

“Thus you shall separate the Levites from among the people of Israel, and the Levites shall be mine. And after that the Levites shall go in to serve at the tent of meeting, when you have cleansed them and offered them as a wave offering. For they are wholly given to me from among the people of Israel. Instead of all who open the womb, the firstborn of all the people of Israel, I have taken them for myself.