It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "I will give this banner to a man who loves Allah and His Messenger, and whom Allah and His Messenger love." So he (peace and blessings be upon him) gave the banner to Sayyiduna Ali (may Allah be pleased with him).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15686