Reference for Bava Batra 345:4

ואי בעית אימא דכולי עלמא אותיות נקנות במסירה והכא בצריך להביא ראיה קא מיפלגי תנא דידן סבר אין צריך להביא ראיה ותנא ברא סבר צריך להביא ראיה

[agree that] 'letters' may be acquired by delivery,<span class="x" onmousemove="('comment',' Mesirah, v. Glos. And no deed of sale is necessary (v. supra 77a). ');">¹⁰</span> but they differ here on [the question whether] it is necessary<span class="x" onmousemove="('comment',' For the holder of the bond. ');">¹¹</span> to produce proof.<span class="x" onmousemove="('comment',' That he received the bond as a gift or purchase and that he did not merely find it or receive it as a deposit. ');">¹²</span>

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