Related for Bava Batra 252:13

אמר ר' אמי טומטום שנקרע ונמצא זכר אינו נוטל פי שנים דאמר קרא (דברים כא, טו) והיה הבן הבכור לשניאה עד שיהא בן משעת הויה

A certain [person once] came before Rabbah b. Bar Hana [and] said to him, 'I am certain that this [man] is a firstborn'. He said to him: 'Whence do you know [this]?' 'Because his father called him foolish<span class="x" onmousemove="('comment',' The witness assumed that 'foolish firstborn' implied that he was 'firstborn to his father' and 'weak in intellect'. ');">³⁶</span> firstborn' 'He might have been the firstborn of his mother [only], because the firstborn of a mother is also called foolish firstborn.'<span class="x" onmousemove="('comment',' 'Foolish', implying that he has the title 'firstborn' without the rights and privileges attached to it. ');">³⁷</span> A certain [person once] came before R. Hanina [and] said to him, 'I am certain that this [man] is firstborn'. He said to him, 'Whence do you know [this]?' — [The other] replied to him: 'Because when [people] came to his father,<span class="x" onmousemove="('comment',' Complaining of certain pains or eruptions on their bodies. ');">³⁸</span>

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