Midrash su Levitico 15:15

3) (Vayikra 5:7) "one for a sin-offering and one for a burnt-offering": (The dedication of) the sin-offering must precede (the dedication of) the burnt-offering. Alternately: that the burnt-offering be of the same species as the sin-offering (turtle-dove or young pigeon, respectively) (and that) if he separated his sin-offering and died, his heirs bring his burnt-offering. Alternately: What is the intent of "one for a sin-offering and one for a burnt-offering?" I might think that since two (birds) are brought in place of a (lamb) sin-offering, they should both be sin-offerings, it is, therefore, written "one for a sin-offering" — and not two; "one for a burnt-offering" — and not two.

15) (Vayikra 15:15) ("And the Cohein shall offer them up, one as a sin-offering, and the other as a burnt-offering; and the Cohein shall make atonement for him before the L–rd from his discharge.") "And the Cohein shall offer them up, one as a sin-offering, and the other as a burnt-offering": The Cohein must designate them, one for a sin-offering; the other, for a burnt-offering. Whence is it derived that if he (the owner) himself designates them it is valid? From (Vayikra 5:7 and Vayikra 5:8):" one for a sin-offering and one for a burnt-offering. And he shall bring them to the Cohein."

16) "and the Cohein shall make atonement for him before the L–rd from his discharge": (implying that) some zavim bring (the sacrifice) and some do not. How so? If he had three sightings, he brings; if he had two sightings, he does not bring. — But perhaps (the meaning is that) if he had two sightings, he brings; if he had three, he does not bring. — But did not one who had three sightings have two sightings? How, then, am I to understand "and the Cohein shall make atonement for him before the L–rd from his discharge"? Some zavim bring the sacrifice and some do not. How so? If he had three sightings, he brings; if he had two sightings, he does not bring.