Midrash su Levitico 6:18

1) (Vayikra 6:18) ("Speak to Aaron and his sons, saying: This is the law of the sin-offering. In the place where the burnt-offering is slaughtered, there shall the sin-offering be slaughtered, before the L–rd; it is holy of holies.") "This is the law of the sin-offering": This (the sin-offering) does not obtain on a bamah (a temporary altar). "the law of the sin-offering": There is one law for all sin-offerings, that their blood (if it sprinkled onto a garment) requires washing. — Now where is it excluded (from washing that we need a verse to include it)? — Because it is written (Vayikra 6:19): "The Cohein who offers it as a sin-offering shall eat it" and (Vayikra 6:20): "and what shall be sprinkled of its blood upon a garment … shall be washed," I would think that only outer sin-offerings, (which are eaten by the Cohanim) require washing, but not inner sin-offerings, (which are burnt). And (what is more) it would follow a fortiori, viz.: Now if holy of holies (guilt-offerings), which are similar to outer sin-offerings in requiring scouring and rinsing (of the vessels in which they have been cooked), do not require washing (of garments), then inner sin-offerings, which are not similar (to them) in requiring scouring and rinsing, (not being eaten and not being cooked), how much more so should they not require washing. It is, therefore, (to negate this) written "the law of the sin-offering": There is one law for all sin-offerings, that their blood requires washing.

(Cant. 7:3 [2], cont.:) YOUR BELLY IS A HEAP OF WHEAT. R. Johanan said: A HEAP OF WHEAT (hittim, sing.: hittah): This is the book of Leviticus, all of which <concerns> sin offerings (hatta'ot, sing.: hatta'ah) and guilt offerings (asamot).⁷Similarly Cant. R. 7:3:2; PR 10:3. It is <the law of> the sin offering (hitta; cf. Lev. 6:18 [25]). It is <the law of> the guilt offering (asham; cf. Lev. 7:1). Moreover, <like the belly in the middle of the body,> it (i.e., Leviticus) is set in the middle of Torah, and all of it <concerns> sin offerings (i.e., the wheat of Cant. 7:3 [2]) and guilt offerings. Ergo (in Cant. 7:3 [2]): YOUR BELLY (i.e., Leviticus) IS A HEAP OF WHEAT (i.e.., sin offerings). Resh Laqish said: Why is it likened to wheat? It is simply that just as all of these wheat grains are reckoned by measure, so all of Israel was reckoned by number (minyan). The elders, the saints, the sages, and all Israel are reckoned by number (minyan).

2) And (Vayikra 6:18): ("This is the law of the sin-offering. In the place where the burnt-offering is slaughtered, there shall) the sin-offering be slaughtered" is meant to include the idolatry sin-offering (of the individual), which is a sin-offering that is fixed, atones, comes from the flock, and comes for a known sin — though not a male (but a she-goat). Or, the Yom Kippur goat, which is a sin-offering that is male, fixed, atones, and comes from the flock — though not for a known sin. Since they are equally weighted, let them both be included.