After the death of Aaron's two sons - they died when they came before God with strange fire - God spoke to Moses:
"Tell your brother Aaron not to enter into the Holy of Holies, barging inside the curtain that's before the Atonement-Cover on the Chest whenever he feels like it, lest he die, because I am present in the Cloud over the Atonement-Cover.
"This is the procedure for Aaron when he enters the Holy Place: He will bring a young bull for an Absolution-Offering and a ram for a Whole-Burnt-Offering;
he will put on the holy linen tunic and the linen underwear, tie the linen sash around him, and put on the linen turban. These are the sacred vestments so he must bathe himself with water before he puts them on.
Then from the Israelite community he will bring two male goats for an Absolution-Offering and a Whole-Burnt-Offering.
"Aaron will offer the bull for his own Absolution-Offering in order to make atonement for himself and his household.
Then he will set the two goats before God at the entrance to the Tent of Meeting
and cast lots over the two goats, one lot for God and the other lot for Azazel.
He will offer the goat on which the lot to God falls as an Absolution-Offering.
The goat on which the lot for Azazel falls will be sent out into the wilderness to Azazel to make atonement.