Musnad Ahmad 25:28

Ishaq narrated to us, Sufyan narrated to us, from al-A’mash, from Muslim al-Batin, from Sa’id ibn Jubayr, from Ibn Abbas, who said: "When the Prophet, peace and blessings of Allah be upon him, was driven out of Makkah, Abu Bakr said: 'They have driven out their Prophet. Indeed, to Allah we belong and to Him we shall return. They will surely be destroyed.' Then it was revealed: 'Permission [to fight] has been given to those who are being fought, because they were wronged. And indeed, Allah is competent to give them victory' [Al-Hajj: 39]." He said: "So he knew that there would be fighting." Ibn Abbas said: "It is the first verse that was revealed regarding fighting."