Numbers 8:21

18 And in place of the first sons among the children of Israel, I have taken the Levites.

19 And I have given them to Aaron and to his sons, from among the children of Israel, to undertake for them all the work of the Tent of meeting, and to take away sin from the children of Israel so that no evil may come on them when they come near the holy place.

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;