Reference for Bekhorot 36:67
ואי אשמועינן הא בהא קאמר רבי עקיבא דתרוייהו לא ביכרו אבל אחת ביכרה ואחת שלא ביכרה וילדו שני זכרים אימא מודי ליה לר' טרפון
<big><b>GEMARA: </b></big>[All these cases where R'Tarfon and R'Akiba differ] are necessary [to be stated]. For if we had been informed of the first case above,<span class="x" onmousemove="('comment',' Where a ewe begot two males.');"><sup>24</sup></span> [I might have assumed] that in that case R'Akiba held that the claimant must produce the evidence, because two males came from one ewe,<span class="x" onmousemove="('comment',' And as there is a doubt, we say that the claimant must produce the evidence.');"><sup>25</sup></span> but in the case of two ewes which had never previously given birth, and where two animals [a male and a female] were born from one, and one [male] from the other, I might have said that he agrees with R'Tarfon that the animal which came forth singly is much the better one.<span class="x" onmousemove="('comment',' For the reason why this is the strong one is because it came forth without a companion and had more room in emerging; therefore it is undoubtedly the firstling.');"><sup>26</sup></span> And if he had stated only the latter case, I might have assumed that in this case R'Akiba [held that the claimant must produce the evidence], for neither had previously given birth, but where one ewe had given birth and the other had not given birth and they begot two males, I might have said that he agrees with R'Tarfon,