The Lord spoke to Moses after the death of the two sons of Aaron, when they drew near before the Lord and died.
The Lord said to Moses: Tell your brother Aaron not to come just at any time into the sanctuary inside the curtain before the mercy seat that is upon the ark, or he will die; for I appear in the cloud upon the mercy seat.
References for Leviticus 16:2
Thus shall Aaron come into the holy place: with a young bull for a sin offering and a ram for a burnt offering.
He shall put on the holy linen tunic, and shall have the linen undergarments next to his body, fasten the linen sash, and wear the linen turban; these are the holy vestments. He shall bathe his body in water, and then put them on.
He shall take from the congregation of the people of Israel two male goats for a sin offering, and one ram for a burnt offering.
Aaron shall offer the bull as a sin offering for himself, and shall make atonement for himself and for his house.
He shall take the two goats and set them before the Lord at the entrance of the tent of meeting;
and Aaron shall cast lots on the two goats, one lot for the Lord and the other lot for Azazel.
References for Leviticus 16:8
Aaron shall present the goat on which the lot fell for the Lord, and offer it as a sin offering;
but the goat on which the lot fell for Azazel shall be presented alive before the Lord to make atonement over it, that it may be sent away into the wilderness to Azazel.
References for Leviticus 16:10