Numbers 4:21 ASV

¹⁹  but thus do unto them, that they may live, and not die, when they approach unto the most holy things: Aaron and his sons shall go in, and appoint them every one 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  And Jehovah spake unto Moses, saying,

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

²³  from thirty years old and upward until fifty years old shalt thou number them; all that enter in to wait upon the service, to do the work in the tent of meeting.