Sahih al-Bukhari 2942
Narrated Sahl bin Sa`d:
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 `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 (ﷺ) 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 (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."
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ الْقَعْنَبِيُّ، حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ أَبِي حَازِمٍ، عَنْ أَبِيهِ، عَنْ سَهْلِ بْنِ سَعْد ٍ ـ رضى الله عنه ـ سَمِعَ النَّبِيَّ صلى الله عليه وسلم يَقُولُ يَوْمَ خَيْبَرَ ‏"‏ لأُعْطِيَنَّ الرَّايَةَ رَجُلاً يَفْتَحُ اللَّهُ عَلَى يَدَيْهِ ‏"‏‏.‏ فَقَامُوا يَرْجُونَ لِذَلِكَ أَيُّهُمْ يُعْطَى، فَغَدَوْا وَكُلُّهُمْ يَرْجُو أَنْ يُعْطَى فَقَالَ ‏"‏ أَيْنَ عَلِيٌّ ‏"‏‏.‏ فَقِيلَ يَشْتَكِي عَيْنَيْهِ، فَأَمَرَ فَدُعِيَ لَهُ، فَبَصَقَ فِي عَيْنَيْهِ، فَبَرَأَ مَكَانَهُ حَتَّى كَأَنَّهُ لَمْ يَكُنْ بِهِ شَىْءٌ فَقَالَ نُقَاتِلُهُمْ حَتَّى يَكُونُوا مِثْلَنَا‏.‏ فَقَالَ ‏"‏ عَلَى رِسْلِكَ حَتَّى تَنْزِلَ بِسَاحَتِهِمْ، ثُمَّ ادْعُهُمْ إِلَى الإِسْلاَمِ، وَأَخْبِرْهُمْ بِمَا يَجِبُ عَلَيْهِمْ، فَوَاللَّهِ لأَنْ يُهْدَى بِكَ رَجُلٌ وَاحِدٌ خَيْرٌ لَكَ مِنْ حُمْرِ النَّعَمِ ‏"‏‏.‏