Sahih Bukhari Volume 005, Book 059, Hadith Number 521.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 59. Military Expeditions Led by the Prophet (PBUH) (Al-Maghaazi)
Narrated By Sahl bin Sad : On the day of Khaibar, Allah's Apostle said, "Tomorrow I will give this flag to a man through whose hands Allah will give us victory. He loves Allah and His Apostle, and he is loved by Allah and His Apostle." The people remained that night, wondering as to who would be given it. In the morning the people went to Allah's Apostle and everyone of them was hopeful to receive it (i.e. the flag). The Prophet said, "Where is Ali bin Abi Talib?" It was said, "He is suffering from eye trouble O Allah's Apostle." He said, "Send for him." 'Ali was brought and Allah's Apostle spat in his eye and invoked good upon him. So 'Ali was cured as if he never had any trouble. Then the Prophet gave him the flag. 'Ali said "O Allah's Apostle! I will fight with them till they become like us." Allah's Apostle said, "Proceed and do not hurry. When you enter their territory, call them to embrace Islam and inform them of Allah's Rights which they should observe, for by Allah, even if a single man is led on the right path (of Islam) by Allah through you, then that will be better for you than the nice red camels.