(How can ye disbelieve….) [3:101].
Ahmad ibn al-Hasan al-Hiri informed us> Muhammad ibn Ya'qub> al-'Abbas al-Duri> Abu Nu'aym al-Fadl ibn Dukayn> Qays ibn al-Rabi'> al-Agharr> Khalifah ibn Husayn> Abu Nasr> Ibn 'Abbas who said: “The Aws and the Khazraj had a feud in the pre-Islamic period. One day, they mentioned to each other what had happened in that period and this led them to brandish their swords at each others. Upon being informed of what was happening, the Messenger of Allah, Allah bless him and give him peace, went to them and this verse was revealed (How can ye disbelieve, when it is ye unto whom Allah's revelations are recited, and His messenger is in your midst?) up to His saying (And hold fast, all of you together, to the cable of Allah, and do not separate) [3:103]”.
Al-Sharif Isma'il ibn al-Hasan ibn Muhammad ibn al-Husayn al-Naqib informed us> his grandfather Muhammad ibn al-Husayn> Ahmad ibn Muhammad ibn al-Hasan al-Hafiz> Hatim ibn Yunus al-Jurjani> Ibrahim ibn Abi al-Layth> al-Ashja'i> Sufyan> Khalifah ibn Husayn> Abu Nasr> Ibn 'Abbas who said: “The Aws and the Khazraj were one day talking and became angry with each other to the extent that war nearly broke out between them. They had taken arms and marched toward each other. Then the verse (How can ye disbelieve, when it is ye unto whom Allah's revelations are recited) up to His saying (He did save you from it…)”.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou