Leviticus 16:2-4

2The Lord said to Moses, “Tell your brother Aaron that he may not come whenever he wants into the holy place behind the curtain in front of the mercy seat on the ark or else he will die, because I appear in the cloud above the mercy seat.

3“Aaron is to enter the most holy place in this way: with a young bull for a sin offering and a ram for a burnt offering.

4He is to wear a holy linen tunic, and linen undergarments are to be on his body. He is to tie a linen sash around him and wrap his head with a linen turban. These are holy garments; he must bathe his body with water before he wears them.