Numbers 8:26

They minister to their brothers in the tent of meeting by keeping guard , but they shall do no service . Thus shall you do to the Levites in assigning their duties .

They may assist their brothers in performing their duties at the tent of meeting, but they themselves must not do the work. This, then, is how you are to assign the responsibilities of the Levites.”

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. Numbers 7 Books Numbers 9