Reference for Yevamot 237:1
מתני׳ <big><strong>האשה</strong></big> שהלך בעלה וצרתה למדינת הים ובאו ואמרו לה מת בעליך לא תנשא ולא תתייבם עד שתדע שמא מעוברת היא צרתה
<b><i>MISHNAH</i></b>. A WOMAN WHOSE HUSBAND AND RIVAL WENT TO A COUNTRY BEYOND THE SEA, AND TO WHOM PEOPLE CAME AND SAID, 'YOUR HUSBAND IS DEAD', MUST NEITHER MARRY AGAIN<span class="x" onmousemove="('comment',' Since her husband, when he departed, was known to have had no issue. ');"><sup>1</sup></span> NOR CONTRACT LEVIRATE MARRIAGE<span class="x" onmousemove="('comment',' It being possible that her rival had a child from their husband. ');"><sup>2</sup></span> UNTIL SHE HAS ASCERTAINED WHETHER HER RIVAL IS PREGNANT.<span class="x" onmousemove="('comment',' If the rival is found to be pregnant the woman is free to marry again; and if she is not pregnant, levirate marriage or halizah must be performed. ');"><sup>3</sup></span>
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