Sahih al-Bukhari, Hadith 3701 ⓘ

Narrated Sahl bin Sa`d: Allah's Messenger said, "Tomorrow I will give the flag to a man with whose leadership Allah will grant (the Muslim) victory." So the people kept on thinking the whole night as to who would be given the flag. The next morning the people went to Allah's Messenger and every one of them hoped that he would be given the flag. The Prophet said, "Where is `Ali bin Abi Talib?" The people replied, "He is suffering from eye trouble, O Allah's Messenger." He said, "Send for him and bring him to me." So when `Ali came, the Prophet spat in his eyes and invoked good on him, and be became alright as if he had no ailment. The Prophet then gave him the flag. `Ali said, "O Allah's Messenger! Shall I fight them (i.e. enemy) till they become like us?" The Prophet said, "Proceed to them steadily till you approach near to them and then invite them to Islam and inform them of their duties towards Allah which Islam prescribes for them, for by Allah, if one man is guided on the right path (i.e. converted to Islam) through you, it would be better for you than (a great number of) red camels."

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا عَبْدُ الْعَزِيزِ، عَنْ أَبِي حَازِمٍ، عَنْ سَهْلِ بْنِ سَعْدٍ ـ رضى الله عنه ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏ لأُعْطِيَنَّ الرَّايَةَ غَدًا رَجُلاً يَفْتَحُ اللَّهُ عَلَى يَدَيْهِ ‏"‏ قَالَ فَبَاتَ النَّاسُ يَدُوكُونَ لَيْلَتَهُمْ أَيُّهُمْ يُعْطَاهَا فَلَمَّا أَصْبَحَ النَّاسُ، غَدَوْا عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم كُلُّهُمْ يَرْجُو أَنْ يُعْطَاهَا فَقَالَ ‏"‏ أَيْنَ عَلِيُّ بْنُ أَبِي طَالِبٍ ‏"‏‏.‏ فَقَالُوا يَشْتَكِي عَيْنَيْهِ يَا رَسُولَ اللَّهِ‏.‏ قَالَ ‏"‏ فَأَرْسِلُوا إِلَيْهِ فَأْتُونِي بِهِ ‏"‏‏.‏ فَلَمَّا جَاءَ بَصَقَ فِي عَيْنَيْهِ، وَدَعَا لَهُ، فَبَرَأَ حَتَّى كَأَنْ لَمْ يَكُنْ بِهِ وَجَعٌ، فَأَعْطَاهُ الرَّايَةَ‏.‏ فَقَالَ عَلِيٌّ يَا رَسُولَ اللَّهِ أُقَاتِلُهُمْ حَتَّى يَكُونُوا مِثْلَنَا فَقَالَ ‏"‏ انْفُذْ عَلَى رِسْلِكَ حَتَّى تَنْزِلَ بِسَاحَتِهِمْ، ثُمَّ ادْعُهُمْ إِلَى الإِسْلاَمِ، وَأَخْبِرْهُمْ بِمَا يَجِبُ عَلَيْهِمْ مِنْ حَقِّ اللَّهِ فِيهِ، فَوَاللَّهِ لأَنْ يَهْدِيَ اللَّهُ بِكَ رَجُلاً وَاحِدًا خَيْرٌ لَكَ مِنْ أَنْ يَكُونَ لَكَ حُمْرُ النَّعَمِ ‏"‏‏.‏