Leviticus 16:2-4

2and the Lord said to Moses: “Tell Aaron your brother not to come at just any time into the Holy Place inside the veil, before the mercy seat which is on the ark, lest he die; for I will appear in the cloud above the mercy seat.

3“Thus Aaron shall come into the Holy Place: with the blood of a young bull as a sin offering, and of a ram as a burnt offering.

4He shall put the holy linen tunic and the linen trousers on his body; he shall be girded with a linen sash, and with the linen turban he shall be attired. These are holy garments. Therefore he shall wash his body in water, and put them on.