Musnad Ahmad 1013:26

Qutaybah ibn Sa'id narrated to us, Ya'qub ibn 'Abd al-Rahman narrated to us, from Abu Hazim, Sahl ibn Sa'd informed me that the Messenger of Allah (ﷺ) said on the day of Khaybar: "I will certainly give this banner tomorrow to a man through whose hands Allah will grant victory; he loves Allah and His Messenger, and Allah and His Messenger love him." He said: The people spent their night restless, wondering 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. He said: "Where is 'Ali ibn Abi Talib?" They said: "He is complaining of his eyes, O Messenger of Allah." He said: "Send for him." So he was brought, and the Messenger of Allah (ﷺ) spat into his eyes and prayed for him, and he recovered as if he had no pain at all. Then he gave him the banner. 'Ali said: "O Messenger of Allah, shall I fight them until they become like us?" He said: "Proceed at your own pace until you descend into their courtyard, then invite them to Islam and inform them of what is incumbent upon them regarding the right of Allah therein. For by Allah, that Allah guides one man through you is better for you than having red camels."