Sayyiduna al-Harith bin Hassan al-Bakri (may Allah be pleased with him) said: "I came to Madinah and saw the Prophet (peace and blessings be upon him) delivering a sermon; Bilal (may Allah be pleased with him) had a sword hung at his side, and black banners were planted. The Prophet (peace and blessings be upon him) had sent Amr bin al-As (may Allah be pleased with him) on an expedition."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3313