Numbers 8:14-17

14In this way you shall separate the Levites from the rest of the Israelites, and the Levites will belong to Me. 15After you have cleansed them and presented them as a wave offering, they may come to serve at the Tent of Meeting.

16For the Levites have been wholly given to Me from among the sons of Israel. I have taken them for Myself in place of all who come first from the womb, the firstborn of all the sons of Israel. 17For every firstborn male in Israel is Mine, both man and beast. I set them apart for Myself on the day I struck down all the firstborn in the land of Egypt.