Numbers 8:16-26

16 They will be given completely to me from the Israelites; I have taken them for myself instead of the firstborn of every Israelite woman.
17 All the firstborn in Israel -- people or animals -- are mine. When I killed all the firstborn in Egypt, I set the firstborn in Israel aside for myself.
18 But I have taken the Levites instead of all the firstborn in Israel.
19 From all the Israelites I have given the Levites to Aaron and his sons so that they may serve the Israelites at the Meeting Tent. They will help remove the Israelites' sins so they will belong to the Lord and so that no disaster will strike the Israelites when they approach the Holy Place."
20 So Moses, Aaron, and all the Israelites obeyed and did with the Levites what the Lord commanded Moses.
21 The Levites made themselves clean and washed their clothes. Then Aaron presented them as an offering to the Lord. He also removed their sins so they would be pure.
22 After that, the Levites came to the Meeting Tent to work, and Aaron and his sons told them what to do. They did with the Levites what the Lord commanded Moses.
23 The Lord said to Moses,
24 "This command is for the Levites. Everyone twenty-five years old or older must come to the Meeting Tent, because they all have jobs to do there.
25 At the age of fifty, they must retire from their jobs and not work again.
26 They may help their fellow Levites with their work at the Meeting Tent, but they must not do the work themselves. This is the way you are to give the Levites their jobs."
Scripture taken from the New Century Version. Copyright © 1987, 1988, 1991 by Thomas Nelson, Inc. Used by permission. All rights reserved.