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 will give this banner to a man at whose hands Allah Almighty will grant victory." So the people came to the Messenger of Allah (peace and blessings be upon him) early in the morning, each of them hoping that he would give him the banner. He (peace and blessings be upon him) said: "Where is Ali?" It was said: O Messenger of Allah! He is suffering from sore eyes. He (peace and blessings be upon him) said: "Send for him." So he was brought, and the Messenger of Allah (peace and blessings be upon him) applied his saliva to his eyes and supplicated, and they were healed. Then he (peace and blessings be upon him) gave him the banner and said: "Go forward and do not look back until you arrive in their territory, then call them to me." Sayyiduna Ali (may Allah be pleased with him) went forward, then turned and said: O Messenger of Allah! Shall I fight them until they say 'La ilaha illa Allah (there is no god but Allah)'? He (peace and blessings be upon him) said: "Proceed calmly. When you reach them, invite them to say 'La ilaha illa Allah'. For if Allah guides even one man to Islam through you, that is better for you than red camels."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5929