Leviticus 16:2-3

2The 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.

3“When Aaron enters the sanctuary area, he must follow these instructions fully. He must bring a young bull for a sin offering and a ram for a burnt offering.