Numbers 4:20

Don't cut off the tribe of the families of the Kohathites from among the Levites;

but thus do to them, that they may live, and not die, when they approach to the most holy things: Aaron and his sons shall go in, and appoint them everyone to his service and to his burden;

Yahweh spoke to Moses, saying,

Take a census of the sons of Gershon also, by their fathers' houses, by their families;