Reference for Yevamot 217:7
רב אשי אמר היינו טעמא דרבי אלעזר דגזר הני משום יתומה בחיי האב והחזירה הכי נמי מסתברא מדקתני סיפא קטנה שהשיאה אביה ונתגרשה כיתומה בחיי האב והחזירה דברי הכל שאסורה ליבם
R. Ashi said, This is the reason of R. Eleazar:<span class="x" onmousemove="('comment',' v. supra p. 757, n. 3. ');"><sup>20</sup></span> He forbade [the levirate marriage of] these<span class="x" onmousemove="('comment',' The remarried women spoken of in our Mishnah. ');"><sup>21</sup></span> as a preventive measure against the remarriage of an 'orphan' [minor] in her father's lifetime.<span class="x" onmousemove="('comment',' Who, as stated in our Mishnah, may not be married by the levir because she retains the status of a divorcee. ');"><sup>22</sup></span> This<span class="x" onmousemove="('comment',' R. Ashi's explanation. ');"><sup>23</sup></span>