Leviticus 16:1-2

The LORD spoke to Moses after the death of Aaron’s two sons, who died after they entered the LORD’s presence and burned the wrong kind of fire before him. The LORD said to Moses, “Warn your brother, Aaron, not to enter the Most Holy Place behind the inner curtain whenever he chooses; if he does, he will die. For the Ark’s cover—the place of atonement—is there, and I myself am present in the cloud above the atonement cover.