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 on the day of Khaybar: "Tomorrow I will give this banner to a man who loves Allah and His Messenger, and whom Allah and His Messenger love." Then he (peace and blessings be upon him) called Sayyiduna Ali (may Allah be pleased with him) and gave the banner to him.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15688