Leviticus 16:1-2

After the death of Aaron’s two sons—they died when they came before GOD with strange fire—GOD spoke to Moses: “Tell your brother Aaron not to enter into the Holy of Holies, barging inside the curtain that’s before the Atonement-Cover on the Chest whenever he feels like it, lest he die, because I am present in the Cloud over the Atonement-Cover.