Leviticus 16:2-3

2The 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. 3“This is how Aaron is to enter the Most Holy Place: He must first bring a young bull for a sin offering Or purification offering; here and throughout this chapter and a ram for a burnt offering.