Midrasch zu Schemot 7:18

9) "and it shall be acceptable for him to make atonement for him" — with that which effects atonement. And what is it that effects atonement? The blood, as it is written (Vayikra 17:11): "For it is the blood which atones for the soul." This tells me only (of atonement being effected by) the blood of a clean (i.e., undefiled) animal. Whence is it derived (that atonement is likewise effected by) the blood of an unclean animal? When it is written (Shemoth 28:38): "And Aaron shall atone (by means of the headplate [the tzitz] for the sin of the holy things," which sin does he atone for? If that of piggul (abuse of offerings), it is stated in that regard (Shemoth 7:18): "It (the offering) shall not be credited to him." If that of nothar (left-over offerings), it is stated in that regard (Shemoth 7:18): "It shall not be accepted." Which sin does he atone for? That of tumah (defilement), whose like (i.e., tumah) was permitted in a communal offering.