"It is not permissible to shed the blood of a Muslim except in one of three cases: A soul for a soul, a adulterer who has been married, and one who separates leaving his religion." (Using translation from Nasāʾī 4721)  

عَنِ النَّبِيِّ ﷺ قَالَ لَا يَحِلُّ دَمُ مُسْلِمٍ إِلَّا بِإِحْدَى ثَلَاثٍ النَّفْسُ بِالنَّفْسِ وَالثَّيِّبُ الزَّانِي وَالتَّارِكُ دِينَهُ الْمُفَارِقُ  

“A man killed (another) during the time of the Messenger of Allah ﷺ and was referred to the Prophet ﷺ. He handed him over to the victim's next of kin, but the killer said: 'O Messenger of Allah ﷺ, by Allah I did not mean to kill him.' The Messenger of Allah ﷺ said to the next of kin, 'If he is telling the truth and you kill him, you will go to Hell.' So he let him go. He had been tied with a rope, and he went out dragging his rope, so he became known as Dhan-Nis'ah (the one with the rope). (Using translation from Ibn Mājah 2690)  

قُتِلَ رَجُلٌ عَلَى عَهْدِ رَسُولِ اللهِ ﷺ فَرُفِعَ الْقَاتِلُ إِلَى النَّبِيِّ ﷺ فَدَفَعَهُ إِلَى وَلِيِّ الْمَقْتُولِ فَقَالَ الْقَاتِلُ يَا رَسُولَ اللهِ لَا وَاللهِ مَا أَرَدْتُ قَتْلَهُ فَقَالَ رَسُولُ اللهِ ﷺ لِوَلِيِّ الْمَقْتُولِ «أَمَا إِنَّهُ إِنْ كَانَ صَادِقًا ثُمَّ قَتَلَتْهُ دَخَلْتَ النَّارَ» فَخَلَّى سَبِيلَهُ قَالَ وَكَانَ مَكْتُوفًا بِنِسْعَةٍ فَخَرَجَ يَجُرُّ نِسْعَتَهُ فَسُمِّيَ ذَا النِّسْعَةِ اللَّفْظُ لِأَحْمَدَ  

A man who had killed someone was brought to the Messenger of Allah, and he was brought by the heir of the victim. The Messenger of Allah said to him. 'Will you forgive him? He said: No.' He said: 'Will you kill him? He said: 'Yes.' He said: 'Go away.' Then when he went away, he called him back and said: will you forgive him?' He said: 'No.' He said: 'Will you accept the Diyah? He said: 'No.' He said: 'will you kill him? He said: 'Yes.' He said: 'Go away.' Then when he had gone he said: If you forgive him, he will carry your sin and the sin of your companion (the victim)." So he forgave him and let him go." He said: "And I saw him dragging his string." (Using translation from Nasāʾī 4723)   

جِيءَ بِالْقَاتِلِ الَّذِي قَتَلَ إِلَى رَسُولِ اللهِ ﷺ جَاءَ بِهِ وَلِيُّ الْمَقْتُولِ فَقَالَ لَهُ رَسُولُ اللهِ ﷺ «أَتَعْفُو؟» قَالَ لَا قَالَ «أَتَأْخُذُ الدِّيَةَ؟» قَالَ لَا قَالَ «الْقَتْلُ؟» قَالَ نَعَمْ قَالَ «اذْهَبْ» فَلَمَّا ذَهَبَ قَالَ «أَمَا إِنَّكَ إِنْ عَفَوْتَ عَنْهُ فَإِنَّهُ يَبُوءُ بِإِثْمِكَ وَإِثْمِ صَاحِبِهِ» فَعَفَا عَنْهُ فَأَرْسَلَهُ قَالَ فَرَأَيْتُهُ يَجُرُّ نِسْعَتَهُ