Reference for Bava Batra 253:12
הדר אוקי רבא אמורא עליה ודרש דברים שאמרתי לכם טעות הן בידי ברם כך אמרו משום ר' ינאי הוכרו ולבסוף נתערבו כותבין הרשאה זה לזה לא הוכרו אין כותבין הרשאה זה לזה
by his side, and made the following exposition: what have told you was in error; but this, indeed, has been reported in the name of R. Jannai. 'If they<span class="x" onmousemove="('comment',' The two sons of whom it is not known which is the firstborn. ');"><sup>36</sup></span> were identified<span class="x" onmousemove="('comment',' When they came to claim a share in their father's bequeathed estate. ');"><sup>34</sup></span> and afterwards they were exchanged, they may give written authorisation to one another, [if] they were not identified<span class="x" onmousemove="('comment',' At their birth. ');"><sup>37</sup></span> they may not give written authorisation to one another.