Leviticus 8:15

And he slew it; and Moses took the blood, and put it upon the horns of the altar round about with his finger, and purified the altar, and poured the blood at the bottom of the altar, and sanctified it to make reconciliation upon it. The altar of burnt offering, although consecrated to the Lord, was yet in need of the purifying blood, in order to sanctify it for the service of the priests, to cover the sins with which they, as sinful men, would defile it while performing the work of their ministry.