Musnad Ahmad 235:1

Abu Bakr ibn Ayyash narrated to us, he said: Asim ibn Abi al-Najud narrated to us, from al-Harith ibn Hassan al-Bakri, who said: We arrived in Medina, and behold, the Messenger of Allah (may Allah bless him and grant him peace) was on the pulpit, and Bilal was standing before him, with a sword girded before the Messenger of Allah (may Allah bless him and grant him peace). And behold, there were black banners. I asked, "What are these banners?" They said, "Amr ibn al-Aas has arrived from a military expedition."