זוהר בראשית — דף קלז ע”ב

The Zohar explains that Isaac prayed opposite his wife after twenty years, since the Holy One, blessed be He, accepts the prayers of the righteous. Abraham had not prayed for sons, whereas Isaac knew through the secrets of wisdom that Jacob would come from him, without knowing from which wife; therefore he prayed “opposite his wife.” Rabbi Yehuda’s son asks why Isaac loved Esau more, and is answered that each kind is drawn to its own: according to the text, Esau belongs to severe judgment, like Isaac, in connection with “for he had a taste for game.” The text then comments on “The children struggled within her”: Rebecca went to the house of study of Shem and Eber, and the children broke against one another.