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).  

عَنِ النَّبِيِّ ﷺ قَالَ لاَ تُقْتَلُ نَفْسٌ إِلاَّ كَانَ عَلَى ابْنِ آدَمَ الأَوَّلِ كِفْلٌ مِنْهَا  

The Prophet ﷺ said, "After me (i.e. after my death), do not become disbelievers, by striking (cutting) the necks of one another.  

عَنِ النَّبِيِّ ﷺ قَالَ لاَ تَرْجِعُوا بَعْدِي كُفَّارًا يَضْرِبُ بَعْضُكُمْ رِقَابَ بَعْضٍ  

The Prophet ﷺ said during Hajjat-al-Wadaʿ, "Let the people be quiet and listen to me. After me, do not become disbelievers, by striking (cutting) the necks of one another."  

قَالَ النَّبِيُّ ﷺ فِي حَجَّةِ الْوَدَاعِ اسْتَنْصِتِ النَّاسَ لاَ تَرْجِعُوا بَعْدِي كُفَّارًا يَضْرِبُ بَعْضُكُمْ رِقَابَ بَعْضٍ رَوَاهُ أَبُو بَكْرَةَ وَابْنُ عَبَّاسٍ عَنِ النَّبِيِّ ﷺ  

The Prophet ﷺ said, "Al-Kaʿba'ir (the biggest sins) are: To join others (as partners) in worship with Allah, to be undutiful to one's parents," or said, "to take a false oath." (The sub-narrator, Shuʿba is not sure) Muʿadh said: Shuʿba said, "Al-Kaʿba'ir (the biggest sins) are: (1) Joining others as partners in worship with Allah, (2) to take a false oath (3) and to be undutiful to one's parents," or said, "to murder (someone unlawfully).  

عَنِ النَّبِيِّ ﷺ قَالَ الْكَبَائِرُ الإِشْرَاكُ بِاللَّهِ وَعُقُوقُ الْوَالِدَيْنِ أَوْ قَالَ الْيَمِينُ الْغَمُوسُ شَكَّ شُعْبَةُ وَقَالَ مُعَاذٌ حَدَّثَنَا شُعْبَةُ قَالَ الْكَبَائِرُ الإِشْرَاكُ بِاللَّهِ وَالْيَمِينُ الْغَمُوسُ وَعُقُوقُ الْوَالِدَيْنِ أَوْ قَالَ وَقَتْلُ النَّفْسِ  

The Prophet ﷺ said, "The biggest of Al-Kaʿba'ir (the great sins) are (1) to join others as partners in worship with Allah, (2) to murder a human being, (3) to be undutiful to one's parents (4) and to make a false statement," or said, "to give a false witness."  

عَنِ النَّبِيِّ ﷺ قَالَ الْكَبَائِرُ وَحَدَّثَنَا عَمْرٌو حَدَّثَنَا شُعْبَةُ عَنِ ابْنِ أَبِي بَكْرٍ عَنْ أَنَسِ بْنِ مَالِكٍ عَنِ النَّبِيِّ ﷺ قَالَ أَكْبَرُ الْكَبَائِرِ الإِشْرَاكُ بِاللَّهِ وَقَتْلُ النَّفْسِ وَعُقُوقُ الْوَالِدَيْنِ وَقَوْلُ الزُّورِ أَوْ قَالَ وَشَهَادَةُ الزُّورِ  

Messenger of Allah ﷺ sent us (to fight) against Al-Huraqa (one of the sub-tribes) of Juhaina. We reached those people in the morning and defeated them. A man from the Ansar and I chased one of their men and when we attacked him, he said, "None has the right to be worshipped but Allah." The Ansari refrained from killing him but I stabbed him with my spear till I killed him. When we reached (Medina), this news reached the Prophet. He said to me, "O Usama! You killed him after he had said, 'None has the right to be worshipped but Allah?"' I said, "O Messenger of Allah ﷺ! He said so in order to save himself." The Prophet ﷺ said, "You killed him after he had said, 'None has the right to be worshipped but Allah." The Prophet ﷺ kept on repeating that statement till I wished I had not been a Muslim before that day.  

بَعَثَنَا رَسُولُ اللَّهِ ﷺ إِلَى الْحُرَقَةِ مِنْ جُهَيْنَةَ قَالَ فَصَبَّحْنَا الْقَوْمَ فَهَزَمْنَاهُمْ قَالَ وَلَحِقْتُ أَنَا وَرَجُلٌ مِنَ الأَنْصَارِ رَجُلاً مِنْهُمْ قَالَ فَلَمَّا غَشِينَاهُ قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ قَالَ فَكَفَّ عَنْهُ الأَنْصَارِيُّ فَطَعَنْتُهُ بِرُمْحِي حَتَّى قَتَلْتُهُ قَالَ فَلَمَّا قَدِمْنَا بَلَغَ ذَلِكَ النَّبِيَّ ﷺ قَالَ فَقَالَ لِي يَا أُسَامَةُ أَقَتَلْتَهُ بَعْدَ مَا قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ قَالَ قُلْتُ يَا رَسُولَ اللَّهِ إِنَّمَا كَانَ مُتَعَوِّذًا قَالَ أَقَتَلْتَهُ بَعْدَ أَنْ قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ قَالَ فَمَا زَالَ يُكَرِّرُهَا عَلَىَّ حَتَّى تَمَنَّيْتُ أَنِّي لَمْ أَكُنْ أَسْلَمْتُ قَبْلَ ذَلِكَ الْيَوْمِ  

I was among those Naqibs (selected leaders) who gave the Pledge of allegiance to Messenger of Allah ﷺ. We gave the oath of allegiance, that we would not join partners in worship besides Allah, would not steal, would not commit illegal sexual intercourse, would not kill a life which Allah has forbidden, would not commit robbery, would not disobey (Allah and His Apostle), and if we fulfilled this pledge we would have Paradise, but if we committed any one of these (sins), then our case will be decided by Allah.  

إِنِّي مِنَ النُّقَبَاءِ الَّذِينَ بَايَعُوا رَسُولَ اللَّهِ ﷺ بَايَعْنَاهُ عَلَى أَنْ لاَ نُشْرِكَ بِاللَّهِ شَيْئًا وَلاَ نَسْرِقَ وَلاَ نَزْنِيَ وَلاَ نَقْتُلَ النَّفْسَ الَّتِي حَرَّمَ اللَّهُ وَلاَ نَنْتَهِبَ وَلاَ نَعْصِيَ بِالْجَنَّةِ إِنْ فَعَلْنَا ذَلِكَ فَإِنْ غَشِينَا مِنْ ذَلِكَ شَيْئًا كَانَ قَضَاءُ ذَلِكَ إِلَى اللَّهِ  

The Prophet ﷺ said, "Whoever carries arms against us, is not from us."  

عَنِ النَّبِيِّ ﷺ قَالَ مَنْ حَمَلَ عَلَيْنَا السِّلاَحَ فَلَيْسَ مِنَّا رَوَاهُ أَبُو مُوسَى عَنِ النَّبِيِّ ﷺ  

I went to help that man (i.e., ʿAli), and on the way I met Abu Bakra who asked me, "Where are you going?" I replied, "I am going to help that man." He said, "Go back, for I heard Messenger of Allah ﷺ saying, 'If two Muslims meet each other with their swords then (both) the killer and the killed one are in the (Hell) Fire.' I said, 'O Messenger of Allah ﷺ! It is alright for the killer, but what about the killed one?' He said, 'The killed one was eager to kill his opponent."  

ذَهَبْتُ لأَنْصُرَ هَذَا الرَّجُلَ فَلَقِيَنِي أَبُو بَكْرَةَ فَقَالَ: أَيْنَ تُرِيدُ؟ قُلْتُ: أَنْصُرُ هَذَا الرَّجُلَ۔ قَالَ: ارْجِعْ، فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ: «إِذَا الْتَقَى الْمُسْلِمَانِ بِسَيْفَيْهِمَا فَالْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ۔» قُلْتُ: يَا رَسُولَ اللَّهِ، هَذَا الْقَاتِلُ، فَمَا بَالُ الْمَقْتُولِ؟ قَالَ: «إِنَّهُ كَانَ حَرِيصًا عَلَى قَتْلِ صَاحِبِهِ۔»