Leviticus 16:1-4

Now the Lord spoke to Moses after the death of the two sons of Aaron, when they had approached the presence of the Lord and died.

The Lord said to Moses: “Tell your brother Aaron that he shall not enter at any time into the Holy Place inside the veil, before the atoning cover which is on the ark, or he will die; for I will appear in the cloud over the atoning cover.

Aaron shall enter the Holy Place with this: with a bull as a sin offering and a ram as a burnt offering.

He shall put on the holy linen tunic, and the linen undergarments shall be next to his body, and he shall be wrapped about the waist with the linen sash and the linen turban wound around his forehead (these are holy garments). He shall bathe his body in water and put them on.