Reference for Bava Batra 253:13
שלחו ליה בני אקרא דאגמא לשמואל ילמדנו רבינו היו מוחזקין בזה שהוא בכור ואמר אביו על אחר בכור הוא מהו שלח להו כותבין הרשאה
The men of Akra di Agama<span class="x" onmousemove="('comment',' ['The fort of Agama' near Pumbeditha (v. Obermeyer. op. cit, P. 237. n. 3).] ');"><sup>40</sup></span> addressed<span class="x" onmousemove="('comment',' Lit., 'Sent'. ');"><sup>41</sup></span> [the following enquiry] to Samuel: Will our master instruct us [as to] what [is the law in the case] where one was generally held-to be a firstborn son, but his father declared that another [son] was the firstborn?<span class="x" onmousemove="('comment',' Which of the two is entitled to the birthrights ');"><sup>42</sup></span> — He sent to them [the following reply]: 'They may write on an authorisation