Numbers 4:19

4:19 but in order that they will live and not die when they approach the most holy things, do this for them: Aaron and his sons will go in and appoint each man to his service and his responsibility.

Numbers 4:27

4:27 “All the service of the Gershonites, whether carrying loads or for any of their work, will be at the direction of Aaron and his sons. You will assign them all their tasks as their responsibility.

Numbers 8:22

8:22 After this, the Levites went in to do their work in the tent of meeting before Aaron and before his sons. As the Lord had commanded Moses concerning the Levites, so they did.