Numbers 4:18-20

4:18 “Do not allow the tribe of the families of the Kohathites to be cut off from among the Levites; 4:19 but in order that they will live and not die when they approach the most holy things, do this for them: Aaron and his sons will go in and appoint each man to his service and his responsibility. 4:20 But the Kohathites are not to go in to watch while the holy things are being covered, or they will die.”