Numbers 8:23-26

23Now the Lord spoke to Moses, saying,

24“This is what applies to the Levites: from 1twenty-five years old and upward 2they shall enter to perform service in the work of the tent of meeting.

25But at the age of fifty years they shall 3retire from service in the work and not work anymore.

26They may, however, 4assist their brothers in the tent of meeting, 5to fulfill an obligation, but they themselves shall do no work. In this way you shall deal with the Levites in their obligations.”