Hadith: for by Allah, if Allah guides a single person through you, that will be better for you than red camels

عَنْ سَهْلُ بْنُ سَعْدٍ رَضِيَ اللَّهُ عَنْهُ أَنَّ رَسُولَ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ يَوْمَ خَيْبَرَ:
«لَأُعْطِيَنَّ هَذِهِ الرَّايَةَ غَدًا رَجُلًا يَفْتَحُ اللَّهُ عَلَى يَدَيْهِ، يُحِبُّ اللَّهَ وَرَسُولَهُ وَيُحِبُّهُ اللَّهُ وَرَسُولُهُ»، قَالَ: فَبَاتَ النَّاسُ يَدُوكُونَ لَيْلَتَهُمْ أَيُّهُمْ يُعْطَاهَا، فَلَمَّا أَصْبَحَ النَّاسُ غَدَوْا عَلَى رَسُولِ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ كُلُّهُمْ يَرْجُو أَنْ يُعْطَاهَا، فَقَالَ: «أَيْنَ عَلِيُّ بْنُ أَبِي طَالِبٍ؟» فَقِيلَ: هُوَ يَا رَسُولَ اللَّهِ يَشْتَكِي عَيْنَيْهِ، قَالَ: «فَأَرْسِلُوا إِلَيْهِ»، فَأُتِيَ بِهِ فَبَصَقَ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ 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 لَأَنْ يَهْدِيَ اللَّهُ بِكَ رَجُلًا وَاحِدًا، خَيْرٌ لَكَ مِنْ أَنْ يَكُونَ لَكَ حُمْرُ النَّعَمِ».
[صحيح] - [متفق عليه] - [صحيح البخاري: 4210]

Sahl ibn Sa‘d (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said on the Day of Khaybar:
"Verily, I shall give this banner tomorrow to a man at whose hands Allah will grant victory; He loves Allah and His Messenger, and Allah and His Messenger love him." He said: People spent their night conversing and talking as to whom it would be given. In the morning, people went to the Messenger of Allah (may Allah's peace and blessings be upon him), all of them hoping to be given the banner. He said: "Where is ‘Ali ibn Abi Tālib?" It was said: O Messenger of Allah, he is suffering from sore eyes. He said: "Send for him." He was, thus, brought, and the Messenger of Allah (may Allah's peace and blessings be upon him) spat into his eyes and supplicated for him. Thereupon, he recovered as if he had suffered no pain at all, so he gave him the banner. ‘Ali said: O Messenger of Allah, should I fight them until they are like us? He said: "Proceed steadily until you reach their courtyard, then, call them to Islam and inform them of what is obligatory upon them from the rights of Allah therein, for by Allah, if Allah guides a single person through you, that will be better for you than red camels."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 4210]

Explanation

The Prophet (may Allah's peace and blessings be upon him) informed the Companions that Muslims would win victory over the Jews of Khaybar the following day at the hands of a man to whom he would give the banner, which is the flag the army takes as its slogan. Among the traits of this man is that he loves Allah and His Messenger and Allah and His Messenger love him. So, the Companions spent their night conversing and talking about the one who would be given the banner, hoping to win that great honor. In the morning, they went to the Prophet (may Allah's peace and blessings be upon him), each wishing to be the one who would win that honor.
The Prophet (may Allah's peace and blessings be upon him) asked about ‘Ali ibn Abi Tālib.
It was said: He is sick suffering from sore eyes.
So, he (may Allah's peace and blessings be upon him) sent for him, and when they brought him, he spat his honorable saliva into ‘Ali's eyes and supplicated for him. ‘Ali thus recovered from his sickness as if he had suffered no pain at all. Then he gave him the banner and ordered him to proceed gently until approaching the enemy's fortress, and he should then invite them to Islam. In case they respond to him, he should inform them of what is obligatory upon them.
Then, the Prophet (may Allah's peace and blessings be upon him) clarified to ‘Ali the merit of calling people to Allah and the fact that if the preacher succeeds in guiding one single person, that will be better for him than having red camels, which represent the Arabs' most precious property, to possess or give out in charity.