Sahl bin Saad reported,

The Messenger of Allah ﷺ said on the day of Khaybar, "I shall give this banner tomorrow to a man through whose hands Allah will grant victory, one who loves Allah and His Messenger, and whom Allah and His Messenger love."The people spent that night wondering which of them would be given the banner. When morning came, they all went to the Messenger of Allah ﷺ, each hoping to be the one given it.He asked, "Where is Ali bin Ubayy Talib?"They replied, "Messenger of Allah, he is suffering from a complaint in his eyes."He said, "Send for him."So Ali was brought, and the Messenger of Allah ﷺ spat into his eyes and prayed for him. He was healed at once, as though he had never been in pain, and the Prophet ﷺ gave him the banner.Ali asked, "Messenger of Allah, shall I fight them until they become like us?"He replied, "Advance calmly until you reach their territory, then call them to Islam and inform them of what is required of them concerning Allah's rights. By Allah, if Allah guides a single man through you, that is better for you than possessing red camels."

عن سهل بن سعد

أن رسول الله ﷺ قال يوم خيبر: لأعطين هذه الراية رجلا يفتح الله على يديه، يحب الله ورسوله، ويحبه الله ورسوله. قال: فبات الناس يدوكون ليلتهم أيهم يعطاها. قال: فلما أصبح الناس غدوا على رسول الله ﷺ كلهم يرجون أن يعطاها فقال: أين على بن أبي طالب؟ فقالوا: هو يا رسول الله يشتكى عينيه. قال: فأرسلوا إليه، فأتي به، فبصق رسول الله ﷺ في عينيه، ودعا له فبرأ، حتى كأن لم يكن به وجع، فأعطاه الراية، فقال علي: يا رسول الله أقاتلهم حتى يكونوا مثلنا. فقال: انفذ على رسلك حتى تنزل بساحتهم، ثم ادعهم إلى الإسلام وأخبرهم بما يجب عليهم من حق الله فيه، فوالله لأن يهدي الله بك رجلا واحدا خير لك من أن يكون لك حمر النعم.