Numbers 4:27-28

27The men in Gershon's clan must do everything that Aaron and his sons ask them to do. Aaron and his sons must tell them the things that they should carry. And they must tell them everything that they should do.

28That is the work for the Tent of Meeting that the descendants of Gershon must do. Ithamar, the son of Aaron the priest, will tell them the things that they must do.’