Numbers 4:22

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;

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

The LORD spoke to Moses, saying,

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.

“This is the service of the families of the Gershonites, in serving and in bearing burdens:

they shall carry the curtains of the tabernacle and the Tent of Meeting, its covering, the covering of sealskin that is on it, the screen for the door of the Tent of Meeting,