Numbers 4:17-20

ADONAI spoke to Moses and Aaron saying, “You are not to let the families of the tribe of the Kohathites be cut off from among the Levites. Do this for them so that they may live and not die whenever they approach the most holy items–-Aaron and his sons are to go into the Sanctuary and assign each man his job and his responsibility. But the Kohathites are not to go in to look at what is holy, not even momentarily, or they will die.”