Sahih al-Bukhari — Hadith 6484
Narrated by Abdullah ibn Mas'ud
لاَ يَحِلُّ دَمُ امْرِئٍ مُسْلِمٍ يَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنِّي رَسُولُ اللَّهِ إِلاَّ بِإِحْدَى ثَلاَثٍ النَّفْسُ بِالنَّفْسِ وَالثَّيِّبُ الزَّانِي وَالْمَارِقُ مِنَ الدِّينِ التَّارِكُ لِلْجَمَاعَةِ
Translation
The Prophet said: 'The blood of a Muslim who testifies that there is no god but Allah and that I am the Messenger of Allah is not lawful except in three cases: a life for a life, the married adulterer, and the one who abandons his religion and separates from the community.'
Commentary
This hadith establishes the sanctity of human life as a fundamental principle in Islamic law. The default state is absolute inviolability — taking a Muslim's life is forbidden. The three exceptions listed are narrow legal categories that historically required extensive judicial process and evidentiary standards.