Sahih Bukhari Volume 005, Book 057, Hadith Number 051.
Narated By Sahl bin Sad : Allah's Apostle said, "Tomorrow I will give the flag to a man with whose leadership Allah will grant (the Muslim) victory." So the people kept on thinking the whole night as to who would be given the flag. The next morning the people went to Allah's Apostle and every one of them hoped that he would be given the flag. The Prophet said, "Where is Ali bin Abi Talib?" The people replied, "He is suffering from eye trouble, O Allah's Apostle." He said, "Send for him and bring him to me." So when 'Ali came, the Prophet spat in his eyes and invoked good on him, and he became alright as if he had no ailment. The Prophet then gave him the flag. 'Ali said, "O Allah's Apostle! Shall I fight them (i.e. enemy) till they become like us?" The Prophet said, "Proceed to them steadily till you approach near to them and then invite them to Islam and inform them of their duties towards Allah which Islam prescribes for them, for by Allah, if one man is guided on the right path (i.e. converted to Islam) through you, it would be better for you than (a great number of) red camels."