Leviticus 16:2, 15

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.

“Then Aaron must slaughter the first goat as a sin offering for the people and carry its blood behind the inner curtain. There he will sprinkle the goat’s blood over the atonement cover and in front of it, just as he did with the bull’s blood.