Reference for Yevamot 243:9

ואינהו נמי אית להו בבואה דחזו ליה בבואה דבבואה ודלמא לדידהו אית להו בבואה דבבואה אמר רבי חנינא אמר לי יונתן שידא בבואה אית להו בבואה דבבואה לית להו

— A Tanna at the school of R. Ishmael taught that at a time of danger<span class="x" onmousemove="('comment',' When a man, for instance, was cast into a pit and his fate is in the balance. ');">³¹</span> [a letter of divorce] may be written and delivered [to the woman]<span class="x" onmousemove="('comment',' In order to release her thereby from perpetual doubt as to the ultimate fate of her husband and from the perpetual prohibition of marrying again. ');">³²</span> even if [the husband who gave the instructions]<span class="x" onmousemove="('comment',' Calling them out, in the case presumed, from the bottom of the pit. ');">³³</span> is unknown [to the witnesses].<span class="x" onmousemove="('comment',' Who have to execute the mission, v. Git. 66a. Similarly in the case dealt with in our Mishnah. Were not the voice to be relied upon the woman might have to remain all her life bereft of her own husband and unable ever to marry another man. ');">³⁴</span> <b><i>MISHNAH</i></b>. R. AKIBA STATED: WHEN I WENT DOWN TO NEHARDEA TO INTERCALATE<span class="x" onmousemove="('comment',' To add another month. The Hebrew leap year contains thirteen, instead of the usual twelve months. ');">³⁵</span>

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