Reference for Bekhorot 84:15

רבי יוחנן לא ס"ל דרב חסדא

But let R'Johanan also say: Who is the authority that does not take into consideration the view of the last Rabbis [in the Mishnah]! For R'Hisda said: The difference of opinion in the Mishnah relates only to a hermaphrodite, but as regards a tumtum all agree that it is a case of a doubtful animal [as to sex].

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