Musannaf Ibn Abi Shayba 36:408

Ya'la ibn 'Ubayd narrated to us, he said: Muhammad ibn Ishaq narrated to us, from al-Zuhri, from 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah, from Ibn 'Abbas, he said: The Messenger of Allah (may Allah bless him and grant him peace) went out in the year of the Conquest, ten days having passed from Ramadan.