It is narrated from Sayyiduna Sahl bin Sa'd (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 shall give this banner to a man at whose hands Allah Almighty will grant victory." The people kept talking among themselves about who would be given it. He (peace and blessings be upon him) said: "Where is Ali bin Abi Talib?" It was said: "O Messenger of Allah, he has a pain in his eye." The Companions sent for him and he was brought. The Messenger of Allah (peace and blessings be upon him) applied his saliva to his eye and supplicated for him, and it was healed as if he had never had any pain. 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 tell them what right of Allah will be obligatory upon them. If Allah guides someone through you, that is better for you than red camels."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5969