Leviticus 16:1-2

1And Jehovah spoke to Moses after the death of the two sons of Aaron, when they came near before Jehovah and died;

2and Jehovah said to Moses, Speak unto Aaron thy brother, that he come not at all times into the sanctuary inside the veil before the mercy-seat which is upon the ark, that he die not; for I will appear in the cloud upon the mercy-seat.