Leviticus 4:7

And the priest shall put some of the blood up on the horns of the altar of sweet incense before the Lord, which is in the Tabernacle of the Congregation, the horns of the golden altar being used only in this case and when the entire nation was concerned, since the offense was considered especially grave; and shall pour all the blood of the bullock, the great bulk of it, but little haying been used for the ceremonies of the Holy Place, at the bottom of the altar of the burnt offering, which is at the door of the Tabernacle of the Congregation, out in the open court. The sacrificial blood, which was to expiate the sin, was thus placed very prominently before the eyes of the Lord, in order to plead for His forgiveness.