"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."  

عَنْ رَسُولِ اللَّهِ ﷺ قَالَ لاَ يَحِلُّ دَمُ امْرِئٍ مُسْلِمٍ إِلاَّ بِإِحْدَى ثَلاَثٍ النَّفْسُ بِالنَّفْسِ وَالثَّيِّبُ الزَّانِي وَالتَّارِكُ دِينَهُ الْمُفَارِقُ  

"A man was killed during the time of the Messenger of Allah, and the Killer was brought to the Prophet. He handed him over to the heir of the victim, but the killer said: 'O Messenger of Allah, by Allah I did not means to kill him.' The Messenger of Allah said to the next of kin: 'If he is telling the truth and you kill him, you will go to the Fire.' So he let him go. He had been tied with a string and he went out dragging his string, so he became known as Dhul-Nis'ah (the one with the string).  

قُتِلَ رَجُلٌ عَلَى عَهْدِ رَسُولِ اللَّهِ ﷺ فَرُفِعَ الْقَاتِلُ إِلَى النَّبِيِّ ﷺ فَدَفَعَهُ إِلَى وَلِيِّ الْمَقْتُولِ فَقَالَ الْقَاتِلُ يَا رَسُولَ اللَّهِ لاَ وَاللَّهِ مَا أَرَدْتُ قَتْلَهُ فَقَالَ رَسُولُ اللَّهِ ﷺ لِوَلِيِّ الْمَقْتُولِ أَمَا إِنَّهُ إِنْ كَانَ صَادِقًا ثُمَّ قَتَلْتَهُ دَخَلْتَ النَّارَ فَخَلَّى سَبِيلَهُ قَالَ وَكَانَ مَكْتُوفًا بِنِسْعَةٍ فَخَرَجَ يَجُرُّ نِسْعَتَهُ فَسُمِّيَ ذَا النِّسْعَةِ  

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."  

جِيءَ بِالْقَاتِلِ الَّذِي قَتَلَ إِلَى رَسُولِ اللَّهِ ﷺ جَاءَ بِهِ وَلِيُّ الْمَقْتُولِ فَقَالَ لَهُ رَسُولُ اللَّهِ ﷺ أَتَعْفُو قَالَ لاَ قَالَ أَتَقْتُلُ قَالَ نَعَمْ قَالَ اذْهَبْ فَلَمَّا ذَهَبَ دَعَاهُ قَالَ أَتَعْفُو قَالَ لاَ قَالَ أَتَأْخُذُ الدِّيَةَ قَالَ لاَ قَالَ أَتَقْتُلُ قَالَ نَعَمْ قَالَ اذْهَبْ فَلَمَّا ذَهَبَ قَالَ أَمَا إِنَّكَ إِنْ عَفَوْتَ عَنْهُ فَإِنَّهُ يَبُوءُ بِإِثْمِكَ وَإِثْمِ صَاحِبِكَ فَعَفَا عَنْهُ فَأَرْسَلَهُ قَالَ فَرَأَيْتُهُ يَجُرُّ نِسْعَتَهُ