صحيح مسلم

Sahl b. Sa'd reported that Allah's Messenger (ﷺ) said on the Day of Khaibar: I would certainly give this standard to a person at whose hand Allah would grant victory and who loves Allah and His Messenger and Allah and His Messenger love him also. The people spent the night thinking as to whom it would be given. When it was morning the people hastened to Allah's Messenger (ﷺ) all of them hoping that that would be given to him. He (the Holy Prophet) said: Where is 'Ali b. Abu Talib? They said: Allah's Messenger, his eyes are sore. He then sent for him and he was brought and Allah's Messenger (ﷺ) applied saliva to his eyes and invoked blessings and he was all right, as if he had no ailment at all, and conferred upon him the standard. 'Ali said: Allah's Messenger, I will fight them until they are like us. Thereupon he (the Holy Prophet) said: Advance cautiously until you reach their open places, thereafter invite them to Islam and inform them what is obligatory for them from the rights of Allah, for, by Allah, if Allah guides aright even one person through you that is better for you than to possess the most valuable of the camels.