Reference for Chullin 149:36

כי פליגי היכא דהושיט ידו למעי בהמה ותלש חלב של בן ט' חי ואכל

[It was taught:] Just as 'the fat and the two kidneys' referred to in the case of the guilt-offering precludes that of a foetus,<span class="x" onmousemove="('comment',' The guilt-offering had to be a male animal, hence the fat mentioned with regard to it which was to be offered upon the altar (cf. Lev. VII, 3, 4) cannot include that of a foetus found in the womb of the animal offered.');">²¹</span>

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