Parshas Lech Licha: The Ramban Tells Us Why We Have Trouble With Yishmael
The pasuk says that Sarah was bothered when Hagar had a child from Avram and she did not. The pasuk tells us that Avram said , "Do what you want with her." Sarah afflicted Hagar by working her with harshness and Hagar ran away. The Ramban says, "Sarah sinned by afflicting Hagar, and so did Avram - by allowing Sarah to do this- therefore Hashem gave her a son who will be a Pereh Adam and will afflict the children of Avram and Sarah will all kinds of afflictions."