Arabs descend from Yishmael who was Abraham's son from Hagar who was Sarah's maidservant (and also from Essau, the wicked child of Isaac). Hagar practiced idolatry after she bore Yishmael and acted wickedly so Sarah told Abraham to kick her out so she won't have a bad influence on Yitzchak. Abraham at first didn't want to listen to Sarah to throw Hagar out but then G-d appeared to Abraham and told him that Sarah was right so he cast away Hagar and her child. Abraham also had several other children as stated in the Bible but G-d told Abraham that Issac would be considered his seed and inherit the land of Israel and the blessings given to Abraham and the Jewish people will descend from Issac.