Reference for Bava Batra 47:1

בשביל של כרמים דאם איתא דמעלמא אתי כיון דמידדי לא מצי אתי דכל דמידדי והדר חזי ליה לקיניה מידדי ואי לא לא מידדי

with a path between vineyards; for though [there is ground for saying that] it came from a distance. [because it is more than fifty cubits from a cote],<span class="x" onmousemove="('comment',' The vines having enabled it to hop further than it would otherwise be able to do. ');">¹</span> yet here, since it can only hop, it cannot have come from a distant cote, because a bird will only hop away from the cote so long as it can still see the cote on turning round, but no further.

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