Reference for Yevamot 219:7
אמר ליה רבינא לרב אשי תינח דקדיש בכספא קדיש בביאה מאי שויוה רבנן לבעילתו בעילת זנות אמר רב יהודה אמר שמואל הלכה כרבי אליעזר וכן אמר רבי אלעזר הלכה כרבי אליעזר:
R. Ashi replied: He<span class="x" onmousemove="('comment',' The second man. ');"><sup>27</sup></span> acted improperly<span class="x" onmousemove="('comment',' In snatching away another man's wife. ');"><sup>28</sup></span> they, therefore, treated him also improperly, and deprived him of the right of valid betrothal.<span class="x" onmousemove="('comment',' All betrothals are made 'in accordance with the law of Moses and Israel' (cf. P.B. p. 298) i.e.. the Pentateuchal, as well as Rabbinic law; hence it is within the power of the Rabbinical authorities to declare certain betrothals, such, for instance, as the present one where the girl was improperly snatched away, to be invalid. ');"><sup>29</sup></span> Said Rabina to R. Ashi: [Your explanation is] satisfactory where the man betrothed [her] with money;<span class="x" onmousemove="('comment',' One of the forms of kinyan in marriage (cf. Kid. 2a). Since the Rabbis are empowered to confiscate a man's property they might well dispose of the money of the betrothal by treating it as a mere gift to the girl. ');"><sup>30</sup></span> what [however, can be said where] he betrothed her by cohabitation? — The Rabbis have declared his cohabitation to be an act of mere fornication.<span class="x" onmousemove="('comment',' Which has no legal validity to effect a kinyan. ');"><sup>31</sup></span>