"It is not permissible to kill a Muslim except in one of three cases: A adulterer who has been married, who is to be stoned; a man who kills a Muslim deliberately; and a man who leaves Islam and wages war against Allah, the Mighty and Sublime, and His Messenger, who is to be killed, crucified or banished from the land."
Hadith Text & Reference
"It is not permissible to kill a Muslim except in one of three cases: An adulterer who has been married, who is to be stoned; a man who kills a Muslim deliberately; and a man who leaves Islam and wages war against Allah, the Mighty and Sublime, and His Messenger, who is to be killed, crucified or banished from the land."
Source: Sunan an-Nasa'i 4743 | Book: The Book of Oaths (qasamah), Retaliation and Blood Money
Scholarly Commentary (Tafsir)
This hadith establishes the fundamental principle of the sanctity of Muslim life, which is inviolable except for these three grave crimes that undermine the very foundations of Islamic society. The prohibition is absolute, making any other killing a major sin requiring severe punishment in this world and the Hereafter.
First Case - The Married Adulterer (Muhsan): This refers to a Muslim who is free, adult, sane, and has entered a valid marriage and had the opportunity for lawful sexual relations. The punishment of stoning (rajm) applies only when the act is proven through confession or the testimony of four righteous witnesses who directly witnessed penetration. This severe punishment protects lineage (nasab) and family structure.
Second Case - Intentional Murder: This applies to one who deliberately kills a Muslim without right. The prescribed punishment is qisas (retaliation), where the legal guardian of the victim has the right to seek equivalent punishment, or they may choose to accept blood money (diyah) or grant forgiveness, with forgiveness being superior in reward.
Third Case - Apostasy with Hostility (Muharib): This is not mere apostasy but apostasy combined with "waging war against Allah and His Messenger" - meaning actively fighting against the Muslim community and its institutions. The ruler (Imam) has discretion in implementing one of the mentioned punishments based on what best serves public interest and deters such aggression against the Islamic state.
Legal Principles Derived
The hadith establishes that life is sacred and protected by divine law, with execution being an exceptional measure reserved only for crimes that threaten the entire social order.
Each case requires strict evidential standards and proper Islamic judicial procedures - these are not matters for individual vigilante justice but must be adjudicated by a qualified Islamic court.
The three exceptions balance between preserving life, maintaining social order, protecting religion, and ensuring justice for victims of the most heinous crimes.