Sayyiduna Ka'b bin Malik (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) passed by him while he was reciting these verses of poetry: "Has news of us reached Ghassan, though between us and them lies a vast stretch of wilderness, around which (weapons) clatter? We fight with swords, for the sake of our sanctity, every black (dense) army, with (further troops) behind it, among which helmets gleam." The Prophet (peace and blessings be upon him) said: "No, O Ka'b bin Malik!" So Ka'b (may Allah be pleased with him) said: "We fight with swords, for the sake of our religion, every black army." The Prophet (peace and blessings be upon him) said: "Yes, O Ka'b!"
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16242