Leviticus 16:2-4

2The Lord said to Moses, “Tell Aaron your brother that he must not enter at any time into the Holy Place inside the veil (the Holy of Holies), before the mercy seat which is on the ark, or he will die, for I will appear in the cloud over the mercy seat.

3Aaron [as high priest] shall enter the Holy Place in this way: with [the blood of] a young bull as a sin offering and [the blood of] a ram as a burnt offering.

4He shall put on the holy linen tunic, and the linen undergarments shall be next to his body, and he shall be belted with the linen sash, and dressed with the linen turban (these are the holy garments). He shall bathe his body in water and put them on.