Rashi on Numbers 4:27/רש"י על במדבר ד׳:כ״ז

1על פי אהרן ובניו AT THE APPOINTMENT OF AARON AND HIS SONS [SHALL BE ALL THE SERVICE OF THE SONS OF THE GERSHONITES] — and which of his sons shall be appointed over them? Everything was (v. 28) “under the hand of Ithamar the son of Aaron the priest”.