Numbers 4:19-20

19Do not let the men in that clan die. That will happen if they come near to the most holy things. You must do this: Aaron and his sons must go into the Tent of Meeting. They will tell each man the work that he must do. And they must tell each man the things that he must carry.

20But the men from Kohath's clan must not look at anything that is holy, even for a moment. If they do look at those things, they will die.’