Leviticus 16:1-2

And the LORD spake unto Moses after the death of the two sons of Aaron, when they offered before the LORD, and died; And the LORD said unto Moses, Speak unto Aaron thy brother, that he come not at all times into the holy {place} within the vail before the mercy seat, which {is} upon the ark; that he die not: for I will appear in the cloud upon the mercy seat.