Numbers 8:14-16

In this way you are to set the Levites apart from the other Israelites, and the Levites will be mine. “After you have purified the Levites and presented them as a wave offering, they are to come to do their work at the tent of meeting. They are the Israelites who are to be given wholly to me. I have taken them as my own in place of the firstborn, the first male offspring from every Israelite woman.