Rashi on Leviticus 9:2/רש"י על ויקרא ט׳:ב׳

1קח לך עגל TAKE THEE A CALF — This animal was selected as a sin offering to announce to him that the Holy One, blessed be He, granted him atonement by means of this calf for the incident of the golden calf which he had made (Midrash Tanchuma, Shmini 4)