Leviticus 16:2

2And the Lord said to Moses: “Tell your brother Aaron not to enter freely into the Most Holy Place16:2 Or the Holy Place; also in verses 16, 17, 20, 23, and 27 behind the veil in front of the mercy seat16:2 Or atonement cover; here and throughout this chapter on the ark, or else he will die, because I appear in the cloud above the mercy seat.