It was said to the Prophet (ﷺ) "Would that you see `Abdullah bin Ubai." So, the Prophet (ﷺ) went to him, riding a donkey, and the Muslims accompanied him, walking on salty barren land. When the Prophet (ﷺ) reached `Abdullah bin Ubai, the latter said, "Keep away from me! By Allah, the bad smell of your donkey has harmed me." On that an Ansari man said (to `Abdullah), "By Allah! The smell of the donkey of Allah's Messenger (ﷺ) is better than your smell." On that a man from `Abdullah's tribe got angry for `Abdullah's sake, and the two men abused each other which caused the friends of the two men to get angry, and the two groups started fighting with sticks, shoes and hands. We were informed that the following Divine Verse was revealed (in this concern):-- "And if two groups of Believers fall to fighting then, make peace between them." (49.9)

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا مُعْتَمِرٌ، قَالَ سَمِعْتُ أَبِي أَنَّ أَنَسًا ـ رضى الله عنه ـ قَالَ قِيلَ لِلنَّبِيِّ صلى الله عليه وسلم لَوْ أَتَيْتَ عَبْدَ اللَّهِ بْنَ أُبَىٍّ. فَانْطَلَقَ إِلَيْهِ النَّبِيُّ صلى الله عليه وسلم وَرَكِبَ حِمَارًا، فَانْطَلَقَ الْمُسْلِمُونَ يَمْشُونَ مَعَهُ، وَهْىَ أَرْضٌ سَبِخَةٌ، فَلَمَّا أَتَاهُ النَّبِيُّ صلى الله عليه وسلم فَقَالَ إِلَيْكَ عَنِّي، وَاللَّهِ لَقَدْ آذَانِي نَتْنُ حِمَارِكَ. فَقَالَ رَجُلٌ مِنَ الأَنْصَارِ مِنْهُمْ وَاللَّهِ لَحِمَارُ رَسُولِ اللَّهِ صلى الله عليه وسلم أَطْيَبُ رِيحًا مِنْكَ. فَغَضِبَ لِعَبْدِ اللَّهِ رَجُلٌ مِنْ قَوْمِهِ فَشَتَمَا، فَغَضِبَ لِكُلِّ وَاحِدٍ مِنْهُمَا أَصْحَابُهُ، فَكَانَ بَيْنَهُمَا ضَرْبٌ بِالْجَرِيدِ وَالأَيْدِي وَالنِّعَالِ، فَبَلَغَنَا أَنَّهَا أُنْزِلَتْ {وَإِنْ طَائِفَتَانِ مِنَ الْمُؤْمِنِينَ اقْتَتَلُوا فَأَصْلِحُوا بَيْنَهُمَا}.

Reference	: Sahih al-Bukhari 2691
In-book reference	: Book 53, Hadith 2
USC-MSA web (English) reference	: Vol. 3, Book 49, Hadith 856
(deprecated numbering scheme) http://sunnah.com/bukhari/53