On the day of Khaibar, Messenger of Allah ﷺ said, "Tomorrow I will give this flag to a man through whose hands Allah will give us victory. He loves Allah and His Apostle, and he is loved by Allah and His Apostle." The people remained that night, wondering as to who would be given it. In the morning the people went to Messenger of Allah ﷺ and everyone of them was hopeful to receive it (i.e. the flag). The Prophet said, "Where is ʿAli bin Abi Talib?" It was said, "He is suffering from eye trouble O Allah's Apostle." He said, "Send for him." ʿAli was brought and Messenger of Allah ﷺ spat in his eye and invoked good upon him. So ʿAli was cured as if he never had any trouble. Then the Prophet ﷺ gave him the flag. ʿAli said "O Messenger of Allah ﷺ! I will fight with them till they become like us." Messenger of Allah ﷺ said, "Proceed and do not hurry. When you enter their territory, call them to embrace Islam and inform them of Allah's Rights which they should observe, for by Allah, even if a single man is led on the right path (of Islam) by Allah through you, then that will be better for you than the nice red camels. (Using translation from Bukhārī 4210)  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ يَوْمَ خَيْبَرَ «لَأُعْطِيَنَّ هَذِهِ الرَّايَةَ غَدًا رَجُلًا يَفْتَحُ اللهُ عَلَى يَدَيْهِ يُحِبُّ اللهَ وَرَسُولَهُ وَيُحِبُّهُ اللهُ وَرَسُولُهُ» فَلَمَّا أَصْبَحَ النَّاسُ غَدَوْا عَلَى رَسُولِ اللهِ ﷺ كُلُّهُمْ يَرْجُو أَنْ يُعْطَاهَا فَقَالَ «أَيْنَ عَلِيُّ بْنُ أَبِي طَالِبٍ؟» فَقِيلَ هُوَ يَا رَسُولَ اللهِ يَشْتَكِي عَيْنَيْهِ فَأَرْسَلُوا إِلَيْهِ فَأُتِيَ بِهِ فَبَصَقَ رَسُولُ اللهِ ﷺ فِي عَيْنَيْهِ وَدَعَا لَهُ فَبَرَأَ حَتَّى كَأَنْ لَمْ يَكُنْ بِهِ وَجَعٌ فَأَعْطَاهُ الرَّايَةَ فَقَالَ عَلِيٌّ يَا رَسُولَ اللهِ «أُقَاتِلُهُمْ حَتَّى يَكُونُوا مِثْلَنَا؟» قَالَ «انْفُذْ عَلَى رِسْلِكَ حَتَّى تَنْزِلَ بِسَاحَتِهِمْ ثُمَّ ادْعُهُمْ إِلَى الْإِسْلَامِ وَأَخْبِرْهُمْ بِمَا يَجِبُ عَلَيْهِمْ مِنْ حَقِّ اللهِ فِيهِ فَوَاللهِ لَأَنْ يَهْدِيَ اللهُ بِكَ رَجُلًا خَيْرٌ لَكَ مِنْ أَنْ يَكُونَ لَكَ حُمُرُ النَّعَمِ»