What is this, the proclamation of the days of jahiliya (ignorance)? They said: Allah's Messenger, there is nothing serious. The two young men fell into dispute and the one struck at the back of the other. Thereupon he (the Holy Prophet) said: Well, a person should help his brother whether he is an oppressor or an oppressed. If he is the oppressor he should prevent him from doing it, for that is his help; and if he is the oppressed he should be helped (against oppression).
Sahih Muslim Book 45, Hadith 80
← Previous Hadith Allah, the Exalted and Glorious, grants respite to the oppessor. But when He lays Hand upon him, He does not then let him off. Re (the Holy Prophet) then recited this verse:" Such is the chastisement of thy Lord when He chastises the towns (inhabited by) wrongdoing persons. Surely, His punishment is painful, severe" (xi. 103).

Sahih Muslim Book 45, Hadith 79 Next Hadith → 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.

Sahih Muslim Book 45, Hadith 81