Reference for Bava Batra 268:1

ליורשו פשיטא לפטור את אשתו מן היבום אצטריכא ליה

As regards the right of heirship! Is it not obvious [that a father is believed]?<span class="x" onmousemove="('comment',' For, were he not his real son there was no need for the father falsely to declare him as an heir. He could have assigned the estate to him as a gift. ');">¹</span> — [The statement] was required in respect of the exemption of his wife from levirate marriage. Surely, this also has been taught [elsewhere]:<span class="x" onmousemove="('comment',' Kid. 64a. ');">²</span>

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