Reference for Yevamot 61:6
אי נמי גירושין דקיימא ערוה דקא מוכח וצרתה קמצרכת לה חליצה אמרי קמו ביה רבנן בגיטא דגיטא מעליא הוא ואתו לייבומי לצרה מפולת מי קמו בהו רבנן במפולת
But why? Here also it may be said, 'This woman finds herself in the status of permissibility to all,<span class="x" onmousemove="('comment',' v. supra p. 192, n. 12. ');"><sup>15</sup></span> would you forbid her [marriage on the basis] of a doubt? You must not forbid her [on the basis] of a doubt'! And should you suggest that here also the prohibition is due to a restriction, [it may be retorted that] it is a restriction which may result in a relaxation, for should you say that she is to perform the <i>halizah</i> she might also be taken in levirate marriage! — In respect of divorce which is of frequent occurrence the Rabbis enacted a preventive measure;<span class="x" onmousemove="('comment',' That wherever the divorce is doubtful the rival must not perform halizah in order that this performance might not lead also to levirate marriage. ');"><sup>16</sup></span> in respect of the collapse of a house which is not of frequent occurrence the Rabbis did not enact any preventive measure. Or else:<span class="x" onmousemove="('comment',' It may be replied. ');"><sup>17</sup></span>