Hubayrah bin Yarim said that Sayyiduna al-Hasan bin Ali (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) never sent Sayyiduna Ali (may Allah be pleased with him) on any mission without giving him the banner."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2707