Numbers 4:20

17 Yahweh spoke to Moses and to Aaron, saying,

18 Don`t cut off the tribe of the families of the Kohathites from among the Levites;

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 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 shall you number them; all who enter in to wait on the service, to do the work in the tent of meeting.