Leviticus 16:2-4

2and the LORD said unto Moses, Speak unto Aaron thy brother, that he come not at all times into the holy place within the veil, before the mercy-seat which is upon the ark; that he die not: for I will appear in the cloud upon the mercy-seat.

3Herewith shall Aaron come into the holy place: with a young bullock for a sin offering, and a ram for a burnt offering.

4He shall put on the holy linen coat, and he shall have the linen breeches upon his flesh, and shall be girded with the linen girdle, and with the linen mitre shall he be attired: they are the holy garments; and he shall bathe his flesh in water, and put them on.