Reference for Shabbat 250:7

ואזדו לטעמייהו דכי אתא רב דימי א"ר חנינא ואמרי לה א"ר זירא א"ר חנינא פעם אחת הלך רבי למקום אחד ומצא נדבך של אבנים ואמר לתלמידיו צאו וחשבו כדי שנשב עליהן למחר ולא הצריכן רבי למעשה

But it is not [analogous]: there, since he did not fasten it firmly, he really made it as nought.<span class="x" onmousemove="('comment',' Since the pumpkin is not fit for drawing water, as the stone will fall out. But here it is enough for his purpose to place the stone upon the barrel, therefore the stone becomes part of the barrel in virtue of that act. ');">¹²</span> Wherein do they differ? One Master (R. Ammi] holds: An act of labour is required;<span class="x" onmousemove="('comment',' For the stone to count as part of the barrel, and mere placing is not an act of labour. ');">¹³</span>

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