Musnad Ahmad 435:29

Ya'qub and Sa'd narrated to us, they said: My father narrated to us, from Muhammad ibn Ishaq, who said: Abd al-Rahman ibn Hurmuz al-A'raj narrated to me, that al-Abbas ibn Abd Allah ibn Abbas gave his daughter in marriage to Abd al-Rahman ibn al-Hakam, and Abd al-Rahman gave his daughter in marriage to him, and they had set a dowry. Mu'awiyah ibn Abi Sufyan—and he was the Caliph—wrote to Marwan, ordering him to separate them, and he said in his letter: "This is the *shighar* (exchange marriage) which the Messenger of Allah, may Allah bless him and grant him peace, forbade."