Numbers 4:21

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;

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

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.