Hadith: It is unlawful to shed the blood of a Muslim who testifies that there is no god but Allah and that Muhammad is the Messenger of Allah, except in one of three cases

عن عائشة رضي الله عنها قالت: قال رسول الله صلى الله عليه وسلم : "لا يَحِلّ دمُ امرئ مسلم، يشهد أن لا إله إلا الله، وأن محمدًا رسول الله، إلا بإحدى ثلاث: رجل زَنَى بَعْدَ إِحْصَان، فإنه يُرْجَم، ورجل خرج مُحاربًا لله ورسوله، فإنه يُقَتُل، أو يُصلَّبُ، أو يُنفى من الأرض، أو يَقتلَ نفسًا، فيُقتلُ بها".
[صحيح] - [رواه أبو داود والنسائي]

‘Ā’ishah (may Allah be pleased with her) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "It is unlawful to shed the blood of a Muslim who testifies that there is no god but Allah and that Muhammad is the Messenger of Allah, except in one of three cases: a married person who commits adultery, they are to be stoned; one who goes forth to fight against Allah and His Messenger, they are to be killed, crucified, or exiled from the land; or one who commits murder, for which he is killed in retribution."
Authentic hadith - [An-Nasaa’i]

Explanation

This Hadīth highlights the inviolability of the blood of Muslims who testify that there is no god but Allah and that Muhammad is His messenger and do not do any of the acts that constitute a violation of the testimonies of Islam. The Wise Lawgiver advocated the safeguarding and protection of lives. Thus, He made such protection and safeguarding part of His legislation. Also, He declared that the gravest sin after polytheism is the killing of a soul which Allah has forbidden to kill. Hence, it is forbidden to kill a Muslim who affirm the two testimonies unless he commits one of three things: 1. Commit unlawful sexual intercourse after Allah has bestowed chastity upon him by virtue of a valid marriage. 2. Kill someone whose life is protected by the Shariah, out of injustice and aggression. Justice requires that the killer is to be killed in order to deter the transgressors from their aggression. 3. Disassociate himself from the Muslim community and go out in rebellion against Allah and His Messenger by committing highway robbery, scaring people, and spreading corruption on earth. The perpetrator is to be killed, crucified, or exiled so that the people become safe from his evil and transgression. These three categories of people are to be killed, for killing them maintains safety of the religion, lives, and honor.