Abdur Rahman ibn Hurmuz al-A'raj said:
Al-Abbas ibn Abdullah ibn al-Abbas married his daughter to Abdur Rahman ibn al-Hakam, and AbdurRahman married his daughter to him. And they made this (exchange) their dower. Mu'awiyah wrote to Marwan commanding him to separate them. He wrote in his letter: This is the shighar which the Messenger of Allah (ﷺ) has forbidden.