Niddah, Chapter 2, Mishnah 3. The Mishnah has taught that the practice of Jewish women is to use two cloths, called edim, in connection with marital relations: one with which the husband wipes himself afterward, and one with which the wife wipes herself afterward. These cloths are more than a matter of cleanliness. They serve as testimony, since blood discovered upon them may reveal that the woman had in fact become a niddah at the time of relations. Our mishnah now works out the consequences of such a discovery, and it distinguishes between three cases.
"Nimtza al shelo, temei'in vechayavin korban." If blood is found on his cloth, the one with which the husband checked himself, both of them are temei'in, and both are obligated to bring a korban. Blood on his cloth points unmistakably to the woman's blood having been present during relations, which means that they had relations while she had the status of a niddah. Since neither of them was aware of this, the transgression was unintentional, and one who transgresses unintentionally brings a korban chatas. Hence both husband and wife are chayavin korban.
"Nimtza al shelah osyom, temei'in vechayavin korban." If the blood is found on her cloth, the ruling depends on how soon after relations she examined it. If it was osyom, that is, immediately afterward, the same halachah applies: both are temei'in and both are obligated to bring a korban chatas. There is no room to attribute the blood to any other source, since the examination followed directly upon relations.
"Nimtza al shelah le'achar zman, temei'in misafeik, ufturin min hakorban." If the blood is found on her cloth only after an interval of time, the case changes. What counts as such an interval will be defined in the following mishnah. Once time has elapsed, there is an outside possibility that the blood originated elsewhere and not from the moment of relations. Even so, the Mishnah rules that they are temei'in misafeik, treated as tamei out of uncertainty, for we cannot dismiss the likelihood that the blood was there all along. But since the tumah here rests only upon a safeik, they are pturin min hakorban, exempt from bringing a korban chatas. A korban chatas is brought for a transgression that is established, not for one that remains in doubt.
The three cases thus form a clear progression. Blood on his cloth, or on hers examined at once, establishes the facts, with both tumah and korban following. Blood on her cloth after an interval leaves us with a doubt: stringent enough to impose tumah, yet not certain enough to require a korban.