Hubayrah bin Yarim said that Sayyiduna al-Hasan bin Ali (may Allah be pleased with him) said: "The Prophet (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 / 2708