Numbers 8:23

20 All these things Moses and Aaron and the children of Israel did to the Levites; as the Lord gave orders to Moses about the Levites, so the children of Israel did.

21 And the Levites were made clean from sin, and their clothing was washed, and Aaron gave them for a wave offering before the Lord; and Aaron took away their sin and made them clean.

22 And then the Levites went in to do their work in the Tent of meeting before Aaron and his sons: all the orders which the Lord had given Moses about the Levites were put into effect.

23 And the Lord said to Moses,

24 This is the rule for the Levites: those of twenty-five years old and over are to go in and do the work of the Tent of meeting;

25 But after they are fifty years old, they are to give up their work and do no more;

26 But be with their brothers in the Tent of meeting, taking care of it but doing no work. This is what you are to do in connection with the Levites and their work.