Numbers 8:23-26

23Then the Lord spoke to Moses, saying,

24“This is what pertains to the Levites: From twenty-five years old and above one may enter to perform service in the work of the tabernacle of meeting;

25and at the age of fifty years they must cease performing this work, and shall work no more.

26They may minister with their brethren in the tabernacle of meeting, to attend to needs, but they themselves shall do no work. Thus you shall do to the Levites regarding their duties.”