Reference for Bava Batra 284:6

אמר רבי יצחק אמר רבי יוחנן המזכה לעובר לא קנה ואם תאמר משנתינו הואיל ודעתו של אדם קרובה אצל בנו

Mar. the son of R. Joseph, said in the name of Raba: A firstborn son who was born after the death of his father<span class="x" onmousemove="('comment',' I.e., where his widow bore twins or where he left two widows and both bore sons one of whom was the firstborn, ');">¹⁵</span> does not receive a double portion. What is the reason? The All Merciful said, He shall acknowledge,<span class="x" onmousemove="('comment',' Deut. XXI, 17. ');">¹⁶</span> and, surely. he is not [alive] to acknowledge [him]. And the law is in accordance with all those versions which Mar the son of R. Joseph quoted in the name of Raba.

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