Leviticus 16:2-3

2The Most Holy Place in the Tent of Meeting is behind the curtain. The Lord said to Moses, ‘Your brother Aaron cannot go behind the curtain at any time that he wants to. Tell him that. The Lord appears in a cloud over the lid of the Covenant Box. So Aaron will die if he does not obey the Lord.

3This is how Aaron must go into the Most Holy Place. He must bring a young bull for a sin offering. He must also bring a male sheep for a burnt offering.