Numbers 4:17-20

The LORD said to Moses and Aaron, “See that the Kohathite tribal clans are not destroyed from among the Levites. So that they may live and not die when they come near the most holy things, do this for them: Aaron and his sons are to go into the sanctuary and assign to each man his work and what he is to carry. But the Kohathites must not go in to look at the holy things, even for a moment, or they will die.”