Numbers 4:18-22

18“Do not cut off the tribe of the families of the Kohathites from among the Levites;

19but do this in regard to them, that they may live and not die when they approach the most holy things: Aaron and his sons shall go in and appoint each of them to his service and his task.

20But they shall not go in to watch while the holy things are being covered, lest they die.”

21Then the Lord spoke to Moses, saying:

22“Also take a census of the sons of Gershon, by their fathers’ house, by their families.