Abu Dawud Book 5, Hadith Number 2070.

Narated By Mu’awiyah : AbdurRahman ibn Hurmuz al-A’raj said: Al-Abbas ibn Abdullah ibn al-Abbas married his daughter to AbdurRahman 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 Apostle of Allah (PBUH) has forbidden.