ויעתר יצחק לה’ לנוכח אשתו…ויעתר לו (תולדות כה:כא)

Rashi says:” לו ולא לה שאין דומה תפלת צדיק בן צדיק לתפלת צדיק בן רשע לפיכך לו ולא לה” To him and not to her’ — because the prayer of a righteous person who is the child of a wicked person, is not at all comparable to the prayer of a righteous person who is also the child of a righteous person . Therefore, Hashem answered ‘to him and not to her.’” 

Yitzchak, in his great humility, felt unworthy of having his prayers answered on his own merit. Therefore, he davened “לנוכח אשתו”—that Hashem should help them in her merit. Yet the Pasuk states, “ויעתר לו ה’”—Hashem responded to him, in his merit. For “אין דומה תפילת צדיק בן צדיק לתפילת צדיק בן רשע”—there is no comparison between the prayer of a tzaddik the son of a tzaddik and that of a tzaddik the son of a rasha. It was “לו ולא לה”—in his merit, not in the merit of his wife, which he had presumed would bring the yeshuah.