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."
Sahih al-Bukhari Book 56, Hadith 218