Numbers 4:20

The LORD spoke to Moses and to Aaron, saying,

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

but do this 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 everyone to his service and to his burden;

The LORD spoke to Moses, saying,

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

you shall count them from thirty years old and upward until fifty years old: all who enter in to wait on the service, to do the work in the Tent of Meeting.