Numbers 8:23-26

And Jehovah spake unto Moses, saying, This is that which belongeth unto the Levites: from twenty and five years old and upward they shall go in to wait upon the service in the work of the tent of meeting: and from the age of fifty years they shall cease waiting upon the work, and shall serve no more, but shall minister with their brethren in the tent of meeting, to keep the charge, and shall do no service. Thus shalt thou do unto the Levites touching their charges.