Numbers 8:26 in the World English Bible
but shall assist their brothers in the Tent of Meeting, to perform the duty, and shall perform no service. This is how you shall have the Levites do their duties.”
Context
25 And from the age of fifty years they shall cease waiting upon the service thereof, and shall serve no more:
26 But shall minister with their brethren in the tabernacle of the congregation, to keep the charge, and shall do no service. Thus shalt thou do unto the Levites touching their charge.