Exodus 30:2

30:2 Its length is to be a foot and a half and its width a foot and a half; it will be square. Its height is to be three feet, with its horns of one piece with it.

Exodus 30:10

30:10 Aaron is to make atonement on its horns once in the year with some of the blood of the sin offering for atonement; once in the year he is to make atonement on it throughout your generations. It is most holy to the Lord.”