Numbers 4:17-20

The LORD spoke to Moses and Aaron, saying, “Let not the tribe of the clans of the Kohathites be destroyed from among the Levites, but deal thus with them, that they may live and not die when they come near to the most holy things: Aaron and his sons shall go in and appoint them each to his task and to his burden, but they shall not go in to look on the holy things even for a moment, lest they die.”