That he heard the Prophet ﷺ 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 ﷺ 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 ʿAli. Someone informed him that he was suffering from eye-trouble. So, he ordered them to bring ʿAli in front of him. Then the Prophet ﷺ spat in his eyes and his eyes were cured immediately as if he had never any eye-trouble. ʿAli said, "We will fight with them (i.e. infidels) till they become like us (i.e. Muslims)." The Prophet ﷺ 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." (Using translation from Bukhārī 2942)
سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ يَوْمَ خَيْبَرَ لَأُعْطِيَنَّ الرَّايَةَ رَجُلًا يَفْتَحُ اللهُ عَلَى يَدَيْهِ فَبَاتَ النَّاسُ يَدُوكُونَ أَيُّهُمْ يُعْطَاهَا فَلَمَّا أَصْبَحَ النَّاسُ غَدَوْا عَلَى رَسُولِ اللهِ ﷺ كُلُّهُمْ يَرْجُو أَنْ يُعْطَاهَا فَقَالَ رَسُولُ اللهِ ﷺ أَيْنَ عَلِيُّ بْنُ أَبِي طَالِبٍ؟ قَالُوا يَا رَسُولَ اللهِ هُوَ يَشْتَكِي عَيْنَيْهِ فَأَرْسَلَ إِلَيْهِ فَبَصَقَ فِي عَيْنِهِ وَدَعَا لَهُ فَبَرَأَ مَكَانَهُ حَتَّى لَكَأَنَّهُ لَمْ يَكُنْ بِهِ شَيْءٌ فَأَعْطَاهُ الرَّايَةَ فَقَالَ يَا رَسُولَ اللهِ أُقَاتِلُهُمْ حَتَّى يَكُونُوا مِثْلَنَا قَالَ عَلَى رِسْلِكَ انْفُذْ حَتَّى تَنْزِلَ بِسَاحَتِهِمْ ثُمَّ ادْعُهُمْ إِلَى الْإِسْلَامِ وَأَخْبِرْهُمْ بِمَا يَجِبُ عَلَيْهِمْ فِيهِ مِنَ الْحَقِّ فَوَاللهِ لَأَنْ يَهْدِيَ اللهُ بِكَ الرَّجُلَ الْوَاحِدَ خَيْرٌ لَكَ مِنْ حُمْرِ النَّعَمِ