Numbers 4:21 WEB

¹⁹  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;

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

²¹  Yahweh spoke to Moses, saying,

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

²³  from thirty years old and upward until fifty years old shall you number them; all who enter in to wait on the service, to do the work in the Tent of Meeting.