Hud al-Asari (may Allah have mercy on him) said: I heard my grandfather, Sayyiduna Mazidah al-Abdi (may Allah be pleased with him), say: "The Prophet (peace and blessings be upon him) tied the banners of the Ansar and made them yellow."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17945