Leviticus 16:2-4

2And the LORD said to Moses: “Tell your brother Aaron not to enter freely into the Most Holy Place1 behind the veil in front of the mercy seat2 on the ark, or else he will die, because I appear in the cloud above the mercy seat.

3This is how Aaron is to enter the Holy Place: with a young bull for a sin offering and a ram for a burnt offering. 4He is to wear the sacred linen tunic, with linen undergarments. He must tie a linen sash around him and put on the linen turban. These are holy garments, and he must bathe himself with water before he wears them.