Musnad Abu Ya'la al-Mawsili 3:94

Ubayd Allah narrated to us, Fudayl ibn Sulayman al-Numayri narrated to us, Abu Hazim narrated to us, Sahl ibn Sa’d narrated to us, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I will certainly give the banner tomorrow to a man through whose hands Allah will grant victory." He said: The people went early to the Messenger of Allah (peace and blessings of Allah be upon him), all of them hoping that he would give the banner to him. He said: "Where is Ali ibn Abi Talib?" They said: "He is suffering from an eye ailment, O Messenger of Allah." He said: "Call him." So he was brought, and he spat into his eye and prayed for him, and he was healed. Then he gave him the banner. Then he said: "Call Ali." So he came. Then he said: "O Ali, do not turn around until you descend upon the people and call them." He said: "O Messenger of Allah, shall we fight them until they say 'There is no god but Allah'?" He said: "Proceed steadily. When you come to them, call them to Allah. For by Allah, that a man should accept Islam at your hands is better for you than having red camels."