Midrasz do Kapłańska 4:35

3) (Vayikra 4:33): "And he shall place his hand on the head of the sin-offering" — to include for semichah: the sin-offering of the Nazirite, of the leper, and of the defiler of the sanctuary. "And he shall slaughter it as a sin-offering" — expressly as a sin-offering. (Vayikra 4:34): "And the Cohein shall take from the blood of the sin-offering" — expressly for a sin-offering. (Vayikra 4:35): "And the Cohein shall atone for him for his sin" — expressly for his sin. — whence they said: If (in sacrificing a sin-offering), he did not slaughter it, or receive (the blood), or bring it (on the altar), or sprinkle it — to that end — it is pasul.

4) (Vayikra 4:35): "And all its fat he shall remove, as the fat of the lamb is removed from the sacrifice of the peace-offerings." What is removed in the sacrifice of the peace-offerings? The fat that is an even layer, membranous, and easily peeled, and the two kidneys and the lobe. Here, too, (the same is removed).

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.