Numbers 4:27-28

At the appointment of Aaron and his sons shall be all the service of the sons of the Gershonites, in all their burdens, and in all their service: and ye shall appoint unto them in charge all their burdens. This is the service of the families of the sons of Gershon in the tabernacle of the congregation: and their charge shall be under the hand of Ithamar the son of Aaron the priest.