Numbers 8:13-14

8:13 You are to have the Levites stand before Aaron and his sons, and then offer them as a wave offering to the Lord. 8:14 And so you are to separate the Levites from among the Israelites, and the Levites will be mine.

Numbers 8:24-26

8:24 “This is what pertains to the Levites: At the age of twenty-five years and upward one may begin to join the company in the work of the tent of meeting, 8:25 and at the age of fifty years they must retire from performing the work and may no longer work. 8:26 They may assist their colleagues in the tent of meeting, to attend to needs, but they must do no work. This is the way you must establish the Levites regarding their duties.”