Jabir b. Abdullah reported: We were along with Allah's Messenger (ﷺ) in an expedition that a person from amongst the emigrants struck at the back of a person from the Ansir. The Ansiri said: O Ansar! And the Muhijir said: O Emigrants! Thereupon Allah's Messenger (ﷺ) said: What are these proclamations of the Days of Ignorance? They said: Allah's Messenger, a person from the emigrants struck at the back of an Ansari, whereupon he said: It is something disgusting. 'Abdullah b. Ubayy heard it and said: They have indeed done it. By Allah, when we would return to Medina the respectable amongst them (the Ansar) would turn away the mean (the emigrants). Thereupon 'Umar said: Permit me so that I should strike the neck of this hypocrite. But he (the Holy Prophet) said: Leave him, the people may not say that Muhammad kills his companions.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَزُهَيْرُ بْنُ حَرْبٍ، وَأَحْمَدُ بْنُ عَبْدَةَ الضَّبِّيُّ، وَابْنُ أَبِي، عُمَرَ - وَاللَّفْظُ لاِبْنِ أَبِي شَيْبَةَ - قَالَ ابْنُ عَبْدَةَ أَخْبَرَنَا وَقَالَ الآخَرُونَ، حَدَّثَنَا سُفْيَانُ بْنُ، عُيَيْنَةَ قَالَ سَمِعَ عَمْرٌو، جَابِرَ بْنَ عَبْدِ اللَّهِ يَقُولُ كُنَّا مَعَ النَّبِيِّ صلى الله عليه وسلم فِي غَزَاةٍ فَكَسَعَ رَجُلٌ مِنَ الْمُهَاجِرِينَ رَجُلاً مِنَ الأَنْصَارِ فَقَالَ الأَنْصَارِيُّ يَا لَلأَنْصَارِ وَقَالَ الْمُهَاجِرِيُّ يَا لَلْمُهَاجِرِينَ . فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَا بَالُ دَعْوَى الْجَاهِلِيَّةِ " . قَالُوا يَا رَسُولَ اللَّهِ كَسَعَ رَجُلٌ مِنَ الْمُهَاجِرِينَ رَجُلاً مِنَ الأَنْصَارِ . فَقَالَ " دَعُوهَا فَإِنَّهَا مُنْتِنَةٌ " . فَسَمِعَهَا عَبْدُ اللَّهِ بْنُ أُبَىٍّ فَقَالَ قَدْ فَعَلُوهَا وَاللَّهِ لَئِنْ رَجَعْنَا إِلَى الْمَدِينَةِ لَيُخْرِجَنَّ الأَعَزُّ مِنْهَا الأَذَلَّ . قَالَ عُمَرُ دَعْنِي أَضْرِبْ عُنُقَ هَذَا الْمُنَافِقِ فَقَالَ " دَعْهُ لاَ يَتَحَدَّثُ النَّاسُ أَنَّ مُحَمَّدًا يَقْتُلُ أَصْحَابَهُ " .
sunnah.com