Midrash for Yevamot 26:5
ובית שמאי מי כתיב לחוץ חוצה כתיב ובית הלל כיון דכתיב חוצה כמאן דכתיב לחוץ דמי
and the All Merciful said, She shall not marry [unto one not of his kin].<span class="x" onmousemove="('comment',' But only unto her husband's brother (Deut. XXV, 5), which shews that a rival is permitted to the other brothers. ');"><sup>13</sup></span> And Beth Hillel?<span class="x" onmousemove="('comment',' Who prohibit the rival to the brothers, how do they explain this text? ');"><sup>14</sup></span> — They require the text for the exposition which Rab Judah reported in the name of Rab. For Rab Judah stated in the name of Rab: Whence is it deduced that betrothal [by a stranger] is of no validity in the case of a sister-in-law?<span class="x" onmousemove="('comment',' Before halizah had been performed. ');"><sup>15</sup></span>