Numbers 8:23-26

23And the LORD spoke to Moses, saying, 24This is it that which pertaineth to the Levites: from twenty and five years old and upward they shall go in to wait upon the service of the tabernacle of the congregation:1 25And from the age of fifty years they shall cease waiting upon the service of it, and shall serve no more:2 26But shall minister with their brethren in the tabernacle of the congregation, to keep the charge, and shall do no service. Thus shalt thou do to the Levites concerning their charge.