Leviticus 16:1-3

Now the LORD spoke to Moses after the death of the two sons of Aaron, when they offered profane fire before the LORD, and died; and the LORD said to Moses: “Tell Aaron your brother not to come at just any time into the Holy Place inside the veil, before the mercy seat which is on the ark, lest he die; for I will appear in the cloud above the mercy seat. “Thus Aaron shall come into the Holy Place: with the blood of a young bull as a sin offering, and of a ram as a burnt offering.