Musnad Abu Ya'la al-Mawsili 252:18

Suwayd ibn Sa'id narrated to us, 'Abd al-'Aziz ibn Abi Hazim narrated to us, from his father, from Sahl ibn Sa'd, who said: I heard the Messenger of Allah (ﷺ) say on the day of Khaybar: "I will certainly give the banner tomorrow to a man through whose hands Allah will grant victory." The people spent the night discussing which of them would be given it. When the people reached the morning, they went to the Messenger of Allah (ﷺ), all of them hoping that it would be given to him. The Messenger of Allah (ﷺ) said: "Where is 'Ali ibn Abi Talib?" They said: "O Messenger of Allah, he is complaining of his eyes." He ordered that he be called, so he was called. He spat into his eyes and prayed for him, and he was healed on the spot, as if there had been nothing wrong with him. He handed the banner to him. He ('Ali) said: "O Messenger of Allah, on what basis shall I fight them?" He (ﷺ) said: "Proceed steadily until you descend into their courtyard, then invite them to Allah, the Mighty and Majestic, and to His Messenger, until they are like us, and inform them of what is incumbent upon them regarding the rights therein. For by Allah, that Allah guides one man through your guidance is better for you than red camels."