Musnad Abu Ya'la al-Mawsili 252:28

And from Sahl ibn Sa’d that he heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "I will certainly give the banner tomorrow to a man through whose hands Allah will grant victory." He said: The people spent the night speculating about that, wondering which of them would be given it. When the people reached the morning, they went to the Messenger of Allah, may Allah bless him and grant him peace, all of them hoping that it would be given to him. The Messenger of Allah, may Allah bless him and grant him peace, 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 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 gave him the banner, and he said: "O Messenger of Allah, shall we fight them until they are like us?" The Messenger of Allah, may Allah bless him and grant him peace, said: "Proceed steadily. When you descend into their courtyard, invite them to Islam and inform them of what is incumbent upon them regarding the rights of Allah therein. For by Allah, that Allah guides one man through your guidance is better for you than red camels."