Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: There was hostility between the tribes of Aws and Khazraj in the Jahiliyyah (pre-Islamic era). One day, while they were sitting, they began recalling the old matters between them until they became angry, and some of them stood up against others with weapons. So this verse was revealed: "And how can you disbelieve while the verses of Allah are recited to you and His Messenger is among you?" [Al Imran: 101], the whole verse, and the two verses after it, up to "And remember the favour of Allah upon you when you were enemies and He brought your hearts together" [Al Imran: 103].
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12612