Leviticus 16:1-3

The LORD spoke to Moses after the death of the two sons of Aaron who died when they approached the LORD. The LORD said to Moses: “Tell your brother Aaron that he is not to come whenever he chooses into the Most Holy Place behind the curtain in front of the atonement cover on the ark, or else he will die. For I will appear in the cloud over the atonement cover. “This is how Aaron is to enter the Most Holy Place: He must first bring a young bull for a sin offering and a ram for a burnt offering.