Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: When the people of Makkah drove out the Prophet (peace and blessings be upon him), Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) said: "Inna lillahi wa inna ilayhi raji'un (We belong to Allah and to Him we return)," and said: They have driven out their Prophet; they will surely be destroyed. Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Then this verse was revealed: "Permission (to fight) has been given to those against whom war is waged, because they have been wronged; and Allah is indeed able to help them" [Surah al-Hajj: 39]. Sayyiduna Ibn Abbas (may Allah be pleased with them both) used to recite it with the reading "udhina". Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) said: I knew that it meant fighting. And Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: This was the first verse revealed about fighting.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17740