Sayyiduna al-Harith bin Hassan al-Bakri (may Allah be pleased with him) said: "We came to Madinah, and the Messenger of Allah (peace and blessings be upon him) was on the pulpit, and Bilal (may Allah be pleased with him) was standing before him (peace and blessings be upon him) with a sword hung at his side, and suddenly black banners came into view. I asked: What are these banners? I was told: Amr bin al-As (may Allah be pleased with him) has returned from a campaign."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3316