Reference for Bava Batra 254:10

ולר"מ דאמר אדם מקנה דבר שלא בא לעולם יכיר למה לי

R. Nahman b. Isaac said to Raba: According to R. Judah it is correct for Scripture to say, he shall acknowledge,'<span class="x" onmousemove="('comment',' Since from this expression it has been inferred that the father's word is the determining factor in deciding the birthright, though another son was generally recognised as firstborn. ');">⁹</span>

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