Leviticus 16:2-4

2and Jehovah said to Moses, Speak unto Aaron thy brother, that he come not at all times into the sanctuary inside 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.

3In this manner shall Aaron come into the sanctuary: with a young bullock for a sin-offering, and a ram for a burnt-offering.

4A holy linen vest shall he put on, and linen trousers shall be upon his flesh, and he shall be girded with a linen girdle, and with the linen mitre shall he be attired: these are holy garments; and he shall bathe his flesh in water, and put them on.