Numbers 8:13-17

And thou shalt set the Levites before Aaron, and before his sons, and offer them for an offering unto the LORD.

Thus shalt thou separate the Levites from among the children of Israel: and the Levites shall be mine.

And after that shall the Levites go in to do the service of the tabernacle of the congregation: and thou shalt cleanse them, and offer them for an offering.

For they are wholly given unto me from among the children of Israel; instead of such as open every womb, even instead of the firstborn of all the children of Israel, have I taken them unto me.

For all the firstborn of the children of Israel are mine, both man and beast: on the day that I smote every firstborn in the land of Egypt I sanctified them for myself.