Reference for Bava Batra 260:4
שיכול והלא דין הוא ומה בכור שהורע כחו שאינו נוטל בראוי כבמוחזק אמרה תורה לא יוכל לבכר פשוט שיפה כחו שנוטל בראוי כבמוחזק לא כל שכן
If [in the case of'] an ordinary [son], who is privileged to receive [a share] in any prospective [property of his father] as in that which is actually in his possession, the Torah [nevertheless] gave authority to the father to transmit [his estate] to whomsoever he pleases, how much more [should he have this right in the case of] a firstborn, whose rights are impaired in that he does not receive [the portion of the birthright] in prospective property as in that which is actually in the possession [of his father]; hence it was expressly stated, He may not make [the son of the beloved] the firstborn. Then let Scripture say, He may not make [the son of the beloved] the firstborn,<span class="x" onmousemove="('comment',' And this will amply prove that the birthright cannot be transferred. ');"><sup>4</sup></span>