Reference for Bava Metzia 187:16
מתני׳ <big><strong>השואל</strong></big> את הפרה ושאל בעליה עמה או שכר בעליה עמה שאל הבעלים או שכרן ואחר כך שאל את הפרה ומתה פטור שנאמר (שמות כב, יד) אם בעליו עמו לא ישלם
R. Judah b. Tema said: In such a case it is a [valid] divorce. R. Judah b. Tema stated a general rule: That which can never be fulfilled, and he [the husband] stipulates it at the beginning, it is only to repel her,<span class="x" onmousemove="('comment',' I.e., to distress and make her think that he is not divorcing her. ');"><sup>14</sup></span> and is valid. R. Nahman said in Rab's name: The <i>halachah</i> is as R. Judah b. Tema. R. Nahman b. Isaac said: Our Mishnah too proves it,<span class="x" onmousemove="('comment',' That the halachah is so. ');"><sup>15</sup></span> for it states: WHATEVER CAN BE FULFILLED EVENTUALLY, AND IT IS STIPULATED AT THE OUTSET, THE STIPULATION IS VALID. Hence, if it is impossible of fulfilment, the stipulation is null. This proves it.<span class="x" onmousemove="('comment',' Since it is taught anonymously. ');"><sup>16</sup></span> <b><i>MISHNAH</i></b>. IF A MAN BORROWS A COW AND BORROWS OR HIRES ITS OWNER WITH IT,<span class="x" onmousemove="('comment',' I.e., the owner lending his personal service. ');"><sup>17</sup></span> OR IF HE FIRST HIRES THE OWNER AND THEN BORROWS THE COW, AND IT DIES, HE IS NOT RESPONSIBLE, FOR IT IS WRITTEN, BUT IF THE OWNER THEREOF BE WITH IT, HE SHALL NOT MAKE IT GOOD.<span class="x" onmousemove="('comment',' Ex. XXII, 14. ');"><sup>18</sup></span>