Reference for Bekhorot 32:8

אין מקבלין צאן ברזל מישראל מפני שהוא רבית

IF A EWE GAVE BIRTH TO WHAT LOOKED LIKE A KID, OR A KID WHICH GAVE BIRTH TO WHAT LOOKED LIKE A EWE, IT IS EXEMPT FROM [THE LAW OF] THE FIRSTLING BUT IF IT POSSESSES CERTAIN CLEAR MARKS [RESEMBLING THE MOTHER] IT IS LIABLE [TO THE LAW OF THE FIRSTLING].

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