Reference for Makkot 27:9

ואידך ק"ו פריכא הוא מה לנדה שכן גופין מוחלקין

And [the rabbis, who learn re'shia bar re'shia from kares b'achoso] would say that it is a flawed kal v'chomer, because what comparison can you make to niddah which involves separte bodies?

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