Reference for Yevamot 129:7
אמר איהו איזיל אינסיב איתתא ואיבדוק נפשאי אמר רבי אמי אף בזו יוציא ויתן כתובה שאני אומר כל הנושא אשה על אשתו יוציא ויתן כתובה רבא אמר נושא אדם כמה נשים על אשתו והוא דאית ליה למיזיינינהי
and the wife pleads, 'The fault is his',<span class="x" onmousemove="('comment',' Lit., 'from him' (cf. supra n. 12). ');"><sup>17</sup></span> R. Ammi ruled: In private matrimonial affairs<span class="x" onmousemove="('comment',' Lit., 'things which are between him and her'. ');"><sup>18</sup></span> the wife is believed. And what is the reason? — She is in a position to know whether emission is forceful,<span class="x" onmousemove="('comment',' Lit., 'shoots like an arrow', which is an essential in fertilization. V. Hag. 15a. ');"><sup>19</sup></span> but he is not in a position to know it. If the husband states that he intends taking another wife to test his potency.<span class="x" onmousemove="('comment',' To beget children. ');"><sup>20</sup></span> R. Ammi ruled: 'He must in this case also divorce [his present wife] and pay her the amount of her <i>kethubah</i>; for I maintain that whosoever takes in addition to his present wife another one must divorce the former and pay her the amount of her <i>kethubah</i>.' Raba said: A man may marry wives in addition to his first wife; provided only that he possesses the means to maintain them.