Levítico 26:27 Midrash: Mekhilta DeRabbi Yishmael

Rebbi says: If it is forbidden to derive benefit from the burnt bullocks and the burnt he-goats, which do not come to atone for the world (viz. Leviticus 26:27), how much more so is it forbidden to derive benefit from eglah arufah, which does come to atone for the world! This tells me only of its flesh (i.e., that benefit may not be derived from the flesh of an ox that killed, which was slaughtered after judgment had been passed upon it.) Whence do I derive (the same for) its skin? R. Yishmael was wont to say: It follows a fortiori, viz.: If the skin of a sin-offering, whose flesh is permitted in its slaughtering, is forbidden (in the derivation of benefit) in its death, then an ox to be stoned, whose flesh is forbidden in its slaughtering, how much more so should its skin be forbidden in its death! — (No,) this is refuted by an ox which has become tamei, which, although its flesh is forbidden in its slaughtering, its skin is permitted in its death.