Numbers 4:17-20

Then the LORD said to Moses and Aaron, “Do not let the Kohathite clans be destroyed from among the Levites! This is what you must do so they will live and not die when they approach the most sacred objects. Aaron and his sons must always go in with them and assign a specific duty or load to each person. The Kohathites must never enter the sanctuary to look at the sacred objects for even a moment, or they will die.”