Sforno on Genesis 25:21/ספורנו על בראשית כ״ה:כ״א

1לנכח אשתו, even though he had been given an assurance from G’d that he would produce seed, he prayed to G’d that the mother of this child or children should be someone who was meritorious, someone of the calibre of Rivkah who was present and standing opposite him..