Reference for Chullin 143:18

אשה מרגשת בעצמה

And R'Ishmael? - The [uncleanness of the] covering stone and supporting stones is established by tradition.<span class="x" onmousemove="('comment',' The verse (Num. XIX, 15) therefore, according to him, serves to exclude the foetus, since there is a tradition that accounts for the uncleanness of the covering stone and supporting stones.');">¹²</span>

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