Reference for Chullin 257:12

למה לי הכשר

that if a man especially set aside a lump of leaven to be used as a seat, he has thereby nullified it.<span class="x" onmousemove="('comment',' I.e., it no longer counts as leaven and by using it on the Passover one does not transgress the prohibition of Ex. XII, 19, for it is no longer a foodstuff but converted into a seat. As a seat it would contract midras uncleanness (which is a grave uncleanness) if a man that has an issue sat upon it.');">⁸</span>

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