Leviticus 16:1-3

Now two of Aaron’s sons had died while offering incense to the LORD, and after that time the LORD spoke to Moses. The LORD said to him, “Tell your brother Aaron that there are times when he cannot go behind the curtain into the Most Holy Place where the Ark is. If he goes in when I appear in a cloud over the lid on the Ark, he will die. “This is how Aaron may enter the Most Holy Place: Before he enters, he must offer a bull for a sin offering and a male sheep for a whole burnt offering.