That he heard the Prophet on the day (of the battle) of Khaibar ...

Hadith 4:192

Narrated Sahl bin Sad:

That he heard the Prophet on the day (of the battle) of Khaibar saying, "I will give the flag to a person at whose hands Allah will grant victory." So, the companions of the Prophet got up, wishing eagerly to see to whom the flag will be given, and everyone of them wished to be given the flag. But the Prophet asked for 'Ali. Someone informed him that he was suffering from eye-trouble. So, he ordered them to bring 'Ali in front of him. Then the Prophet spat in his eyes and his eyes were cured immediately as if he had never any eye-trouble. 'Ali said, "We will fight with them (i.e. infidels) till they become like us (i.e. Muslims)." The Prophet said, "Be patient, till you face them and invite them to Islam and inform them of what Allah has enjoined upon them. By Allah! If a single person embraces Islam at your hands (i.e. through you), that will be better for you than the red camels."