Leviticus 16:2-4

2The Lord said to Moses, “Warn your brother, Aaron, not to enter the Most Holy Place behind the inner curtain whenever he chooses; if he does, he will die. For the Ark’s cover—the place of atonement—is there, and I myself am present in the cloud above the atonement cover.

3“When Aaron enters the sanctuary area, he must follow these instructions fully. He must bring a young bull for a sin offering and a ram for a burnt offering.

4He must put on his linen tunic and the linen undergarments worn next to his body. He must tie the linen sash around his waist and put the linen turban on his head. These are sacred garments, so he must bathe himself in water before he puts them on.