Numbers 4:17-20

Then the LORD spoke to Moses and Aaron, saying: “Do not cut off the tribe of the families of the Kohathites from among the Levites; but do this in regard to them, that they may live and not die when they approach the most holy things: Aaron and his sons shall go in and appoint each of them to his service and his task. But they shall not go in to watch while the holy things are being covered, lest they die.”