Reference for Bava Batra 263:8

למימרא דלא אזיל רב נחמן בתר אומדנא

and having heard that the latter<span class="x" onmousemove="('comment',' Lit., 'his son'. ');">²⁸</span> had died, assigned all his property, in writing, to strangers; though his son subsequently appeared, his gift is [nevertheless, legally] valid.<span class="x" onmousemove="('comment',' Lit., 'a gift'. Since it was made unconditionally. ');">²⁹</span> R. Simeon b. Menasya said: His gift is not [legally] a gift, for had he known that his son was alive, he would not have given it away.<span class="x" onmousemove="('comment',' Lit., 'written them'. ');">³⁰</span> And R. Nahman said: The <i>halachah</i> is in accordance with R. Simeon b. Menasya!<span class="x" onmousemove="('comment',' As R. Nahman upholds it. Simeon's decision, according to which it is assumed that 'had the father known that his son was alive he would not have made the gift', he most also agree with the view that an assumption is to be taken into consideration. How, then, (v. supra note 5), could R. Nahman say that the widow forfeited the rights of her kethubah? ');">³¹</span>

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