(V.5:27) Narrated ‘Abdullâh رضي الله عنه: The Prophetصلى الله عليه وسلم said, "None (no human being) is killed or murdered (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 Qâbîl)." (Sahih Al-Bukhâri, Vol.9, Hadîth No.6)
Narrated ‘Abdullâh bin ‘Umar رضي الله عنهما: The Prophet صلى الله عليه وسلم said, "After me (i.e., after my death), do not become disbelievers, by striking (cutting) the necks of one another."
(Sahih Al-Bukhârî, Vol.9, Hadîth No.7).