Leviticus 16:2-4

New International Version - UK

² The Lord said to Moses: ‘Tell your brother Aaron that he is not to come whenever he chooses into the Most Holy Place behind the curtain in front of the atonement cover on the ark, or else he will die. For I will appear in the cloud over the atonement cover.

³ ‘This is how Aaron is to enter the Most Holy Place: he must first bring a young bull for a sin offering^[a] and a ram for a burnt offering. ⁴ He is to put on the sacred linen tunic, with linen undergarments next to his body; he is to tie the linen sash round him and put on the linen turban. These are sacred garments; so he must bathe himself with water before he puts them on.