Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say on the day of Khaybar: "I will give this banner to a man at whose hands Allah Almighty will grant victory." The people spent the whole night talking about who would be given it. When morning came, all the people went early to the Messenger of Allah (peace and blessings be upon him), each one hoping that he would be given it. The Messenger of Allah (peace and blessings be upon him) said: "Where is Ali bin Abi Talib?" It was said: O Messenger of Allah, his eyes are sore. He (peace and blessings be upon him) sent for him, applied his saliva to his eyes and supplicated for him, and they were healed at once, as if nothing had been wrong with him. He (peace and blessings be upon him) gave him the banner. Sayyiduna Ali (may Allah be pleased with him) said: O Messenger of Allah, shall I fight them until they become like us? He (peace and blessings be upon him) said: "Proceed calmly until you descend into their territory, then invite them to Islam and inform them of the rights that will be obligatory upon them in it. By Allah, if Allah guides a single man through you, that is better for you than red camels."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5856