Leviticus 16:2-6

and the LORD said unto Moses, Speak unto Aaron thy brother, that he come not at all times into the holy place within the vail before the mercy seat, which is upon the ark; that he die not: for I will appear in the cloud upon the mercy seat. Thus shall Aaron come into the holy place: with a young bullock for a sin offering, and a ram for a burnt offering. He shall put on the holy linen coat, and he shall have the linen breeches upon his flesh, and shall be girded with a linen girdle, and with the linen mitre shall he be attired: these are holy garments; therefore shall he wash his flesh in water, and so put them on. And he shall take of the congregation of the children of Israel two kids of the goats for a sin offering, and one ram for a burnt offering. And Aaron shall offer his bullock of the sin offering, which is for himself, and make an atonement for himself, and for his house.