Leviticus 16:3-4

“This is how Aaron may enter the Most Holy Place: Before he enters, he must offer a bull for a sin offering and a male sheep for a whole burnt offering. He must put on the holy linen inner robe, with the linen underclothes next to his body. His belt will be the cloth belt, and he will wear the linen turban. These are holy clothes, so he must bathe his body in water before he puts them on.