Numbers 8:14-16

In this way, you will set the Levites apart from the rest of the people of Israel, and the Levites will belong to me. After this, they may go into the Tabernacle to do their work, because you have purified them and presented them as a special offering. “Of all the people of Israel, the Levites are reserved for me. I have claimed them for myself in place of all the firstborn sons of the Israelites; I have taken the Levites as their substitutes.