صحيح البخاري

Narrated Sahl:

On the day (of the battle) of Khaibar the Prophet (ﷺ) said, "Tomorrow I will give the flag to somebody who will be given victory (by Allah) and who loves Allah and His Apostle and is loved by Allah and His Apostle." So, the people wondered all that night as to who would receive the flag and in the morning everyone hoped that he would be that person. Allah's Messenger (ﷺ) asked, "Where is `Ali?" He was told that `Ali was suffering from eye-trouble, so he applied saliva to his eyes and invoked Allah to cure him. He at once got cured as if he had no ailment. The Prophet (ﷺ) gave him the flag. `Ali said, "Should I fight them till they become like us (i.e. Muslim)?" The Prophet (ﷺ) said, "Go to them patiently and calmly till you enter the land. Then, invite them to Islam, and inform them what is enjoined upon them, for, by Allah, if Allah gives guidance to somebody through you, it is better for you than possessing red camels."