Numbers 4:21

18 Do not let the family of the Kohathites be cut off from among the Levites;

19 But do this to them, so that life and not death may be theirs when they come near the most holy things; let Aaron and his sons go in and give to every one his work and that which he is to take up;

20 But they themselves are not to go in to see the holy place, even for a minute, for fear of death.

22 Let the sons of Gershon be numbered by families, in the order of their fathers' houses;

23 All those from thirty to fifty years old who are able to do the work of the Tent of meeting.

24 This is to be the work of the Gershonites, the things they are to do and take up.