Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The banner of the Messenger of Allah (peace and blessings be upon him) was arranged this way in every battle: the banner of the Muhajirun was with Sayyiduna Ali bin Abi Talib (may Allah be pleased with him), and the banner of the Ansar was with Sayyiduna Sa'd bin Ubadah (may Allah be pleased with him).
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5337