Majma al-Zawaid — Hadith 13054

Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates: Between the Aws and Khazraj tribes of the Ansar there had been enmity since the time of Ignorance (Jahiliyyah). When the Prophet (peace and blessings be upon him) came, this enmity of theirs ended, and Allah Almighty created affection between their hearts. Once they were sitting in a gathering of theirs, when a man from the Aws tribe recited a verse of poetry in which the Khazraj tribe was satirised; then a man from the Khazraj tribe recited a verse in which the Aws tribe was satirised. They kept reciting such verses until they became ready to fight one another; they took up their weapons and set out for battle. When news of this reached the Prophet (peace and blessings be upon him), revelation came down upon him. He came walking swiftly, having lifted the garment from his shins. When he saw them, he called out to them and recited this verse: "O you who believe, fear Allah Almighty as He deserves to be feared, and when you die, be Muslims." When he had finished reciting the verses, they set their weapons aside and embraced one another, weeping.
Al-Tabarani recorded it in al-Mu'jam al-Saghir. Its chain contains a narrator, Ghassan bin al-Rabi', and he is da'if (weak).