Exodus 30:10

10 aAaron shall make atonement on its horns once a year. With the blood of the bsin offering of atonement he shall make atonement for it once in the year throughout your generations. It is most holy to the Lord.”

Leviticus 16:2

2and the Lord said to Moses, “Tell Aaron your brother not to ccome at any time into the Holy Place inside the veil, before the mercy seat that is on the ark, so that he may not die. For dI will appear in the cloud over the mercy seat.
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