Reference for Yevamot 237:12
אמר רבא מכדי הא דאורייתא והא דאורייתא מה לי איסור כרת מה לי איסור לאו אלא אמר רבא
— It is possible that since the woman was confirmed<span class="x" onmousemove="('comment',' When her mother-in-law departed. ');"><sup>32</sup></span> in her status of permissibility to strangers<span class="x" onmousemove="('comment',' Lit., 'to the market'; because there was no known levir. ');"><sup>33</sup></span> [the possibility of the birth of a levir] was not taken by him<span class="x" onmousemove="('comment',' R. Meir. ');"><sup>34</sup></span> into consideration. In the first clause, then,<span class="x" onmousemove="('comment',' If a woman's confirmed status at a certain period is a determining factor. ');"><sup>35</sup></span> where she was confirmed in the status of eligibility for the levirate marriage,<span class="x" onmousemove="('comment',' Since her husband when he departed, had no issue. ');"><sup>36</sup></span> let her contract the levirate marriage! — R. Nahman replied in the name of Rabbah b. Abbuha: In the first clause where a prohibition which is subject to the penalty of <i>kareth</i> [is involved, the possibility of the birth of a son]<span class="x" onmousemove="('comment',' By the rival. ');"><sup>37</sup></span> had to be provided against; in the final clause, however, where a prohibitory law [only is involved]<span class="x" onmousemove="('comment',' The marriage of a yebamah to a stranger. ');"><sup>38</sup></span> no [such possibility]<span class="x" onmousemove="('comment',' That a son was born by the mother-in-law. ');"><sup>39</sup></span> was taken into consideration. Said Raba: Consider: The one [prohibition] is Pentateuchal and the other also is Pentateuchal;<span class="x" onmousemove="('comment',' Neither is a mere Rabbinically preventive measure. ');"><sup>40</sup></span> what matters it, then, whether the prohibition is one involving <i>kareth</i> or whether it is only a mere prohibitory law? — Rather, said Raba;