Leviticus 16:2-5

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. 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.