Musnad Ahmad 25:508

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: Dawud ibn al-Husayn narrated to me, from 'Ikrimah, from Ibn 'Abbas: "That the Messenger of Allah, peace and blessings of Allah be upon him, returned his daughter Zaynab to Abu al-'As ibn al-Rabi'. Her Islam had preceded his Islam by six years, based on the first marriage contract, and he did not introduce a new testimony or a new dowry."