Leviticus 16:1-4

The LORD spoke to Moses after the death of the two sons of Aaron, when they drew near before the LORD and died, and the LORD said to Moses, “Tell Aaron your brother not to come at any time into the Holy Place inside the veil, before the mercy seat that is on the ark, so that he may not die. For I will appear in the cloud over the mercy seat. But in this way Aaron shall come into the Holy Place: with a bull from the herd for a sin offering and a ram for a burnt offering. He shall put on the holy linen coat and shall have the linen undergarment on his body, and he shall tie the linen sash around his waist, and wear the linen turban; these are the holy garments. He shall bathe his body in water and then put them on.