Sahl bin Saad reported,

The Messenger of Allah ﷺ said on the day of Khaybar, "Tomorrow I will give this banner to a man through whose hands Allah will grant victory. He loves Allah and His Messenger, and Allah and His Messenger love him."The people spent the night wondering which of them would be given the banner. When morning came, they all went eagerly to the Messenger of Allah ﷺ, each hoping to be the one given it.He asked, "Where is Ali bin Abu Talib?"Someone replied, "Messenger of Allah, he is suffering from a complaint in his eyes."He said, "Send for him."When Ali was brought, the Messenger of Allah ﷺ spat into his eyes and prayed for him, and he was healed as though he had never felt any pain. He then gave him the banner.Ali said, "Messenger of Allah, shall I fight them until they become like us?"He replied, "Proceed calmly until you reach their territory, then call them to Islam and inform them of the rights of Allah that are due upon them. By Allah, if Allah guides a single man through you, that is better for you than having red camels."

عن سهل بن سعد

أن رسول الله ﷺ قال يوم خيبر: الأعطين هذه الراية غدا رجلا، يفتح الله على يديه، يحب الله ورسوله، ويحبه الله ورسوله، قال: فبات الناس يدوكون ليلتهم أيهم يعطاها؟ فلما أصبح الناس هدوا على رسول الله ﷺ، كلهم يرجو أن يعطاها، فقال: أين علي بن أبي طالب؟ فقيل: هو يا رسول الله يشتكي 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 حمرالنعم.