Numbers 4:21

19but 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;

20but they shall not go in to see the sanctuary even for a moment, lest they die.”

21The LORD spoke to Moses, saying,

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

23you 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.