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 the banner to a man at whose hands Allah will grant victory." The people spent the night discussing which of them would be given it. In the morning the people went to the Messenger of Allah (peace and blessings be upon him), each hoping that it would be given to him. The Messenger of Allah (peace and blessings be upon him) said: "Where is Ali bin Abi Talib (may Allah be pleased with him)?" The Companions (may Allah be pleased with them) said: O Messenger of Allah, his eyes are troubling him. He (peace and blessings be upon him) sent for him, applied his saliva to his eyes and supplicated for him. He was healed on the spot, as though he had never had any ailment. He (peace and blessings be upon him) gave him the banner, and he said: O Messenger of Allah, shall we fight them until they become like us? He (peace and blessings be upon him) said: "Go gently; proceed until you camp in their open ground, then invite them to Islam and inform them of the obligations upon them in it. By Allah, that Allah should guide a single man through you is better for you than red camels."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18230