Midrasz do Kapłańska 4:31

5) (Vayikra 4:35): "And he shall atone for him," (Vayikra 4:31): "And he shall atone for him," (Vayikra 4:26): "And he shall atone for him." What is the intent of this? (i.e., Why three times?) It is needed. For it might be reasoned: Blood is mentioned (as being applied) below (the red line on the altar [i.e., the blood of burnt-offerings, etc.]), and blood is mentioned (as being applied) above the red line (i.e., the blood of sin-offerings, applied on the four horns). Just as with the blood applied below, one application atones, so, with the blood applied above, one application (on one horn) atones.