Midrasch zu Wajikra 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.

6) Or, go in this direction: Blood is mentioned (as being applied) within (i.e., the blood of inner sin-offerings), and blood is mentioned (as being applied) outside (i.e., the blood of outer sin-offerings). Just as with the blood applied within, if he omitted one of the applications, he does not atone, so with the blood applied outside — if he omits one of the applications he does not atone.

7) Let us see what it is most like. We derive outer blood (that of the sin-offering [from outer blood, that of the burnt-offering]), and this is not to be refuted by inner (blood). Or, go in this direction: We derive "blood of the (outer) sin-offering on the horns" from "blood of the (inner) sin-offering on the horns," and this is not to be refuted by the lower (blood), which is not "blood of the sin-offering on the horns." It is, therefore, written: "And he shall atone for him" — even if he made only three applications (instead of four); "and he shall atone for him" — even if he made only two applications; "and he shall atone for him" — even if he made only one application.