Sahih Bukhari Volume 4, Book 56, Hadith 2943

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 ‏"‏‏

Narrated By Sahl bin Sad : That he heard the Prophet 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 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 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 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."