Leviticus 16:2-4

2The Lord said, “Tell your brother Aaron that he cannot go into the holy place whenever he wants to. If he goes up to the canopy and stands in front of the throne of mercy on the ark, he will die, because I appear in the smoke above the throne of mercy.

3“This is what Aaron must do in order to come into the holy place: He must take a bull as an offering for sin and a ram as a burnt offering.

4He must put on a holy linen robe and wear linen undergarments. He must wear a linen belt and turban. These are holy clothes. He must wash his body and put them on.