Abd al-Rahman bin Hurmuz al-A'raj (may Allah have mercy on him) said that al-Abbas bin Abdullah bin Abbas (may Allah be pleased with them both) married his daughter to Abd al-Rahman bin al-Hakam, and Abd al-Rahman bin al-Hakam married his daughter to him, and they also set a dowry. Sayyiduna Mu'awiyah (may Allah be pleased with him) wrote to Marwan (may Allah have mercy on him) to separate them, because this "is the shighar which the Messenger of Allah (peace and blessings be upon him) forbade".
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14140