Numbers 4:18-22

18Cut ye not off the tribe of the families of the Kohathites from among the Levites:

19but 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:

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

21And the LORD spake unto Moses, saying,

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