Sahih Muslim Book 45, Hadith 82
Leave it. for it is something disgusting. Ibn Mansur said that in the narration transmitted on the authority of Amr (these words are also found):" I heard Jabir."
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.
A believer is like a brick for another believer, the one supporting the other.