וַתֵּ֡לֶד שָׂרָה֩ אֵ֨שֶׁת אֲדֹנִ֥י בֵן֙ לַֽאדֹנִ֔י אַחֲרֵ֖י זִקְנָתָ֑הּ וַיִּתֶּן־לּ֖וֹ אֶת־כָּל־אֲשֶׁר־לֽוֹ׃
And Sarah my master’s wife bore a son to my master when she was old; and unto him hath he given all that he hath.
Rashi on Genesis
ויתן לו את כל אשר לו AND HE HATH GIVEN TO HIM ALL THAT HE HATH — he showed them the deed of gift.
Rashbam on Genesis
ויתן לו את כל אשר לו, his brother will not share in the inheritance with Yitzchok.
Sforno on Genesis
אחרי זקנתו, this is why this son was beloved for him from the day he was born.