Sahih al-Bukhari Book 87, Hadith 7
The Prophet (ﷺ) said, "After me (i.e. after my death), do not become disbelievers, by striking (cutting) the
necks of one another.
The Prophet (ﷺ) said, "No human being is killed unjustly, but a part of responsibility for the crime is laid on the first son of Adam who invented the tradition of killing (murdering) on the earth. (It is said that he was Qabil).
The Prophet (ﷺ) said during Hajjat-al-Wada`, "Let the people be quiet and listen to me. After me, do not become disbelievers, by striking (cutting) the necks of one another."