Hadith: The blood of a Muslim is inviolable except by one of three: adultery by a married person, a life for a life (homicide), abandoning Islam and separating from the Islamic body

عن عبد الله بن مسعود رضي الله عنه قال: قال رسول الله -صلى الله عليه وآله وسلم-: «لا يحل دم امرئ مسلم إلا بإحدى ثلاث: الثَّيِّبُ الزاني، والنفسُ بالنفس، والتاركُ لدينه المفارقُ للجماعة».
[صحيح] - [متفق عليه]

‘Abdullāh ibn Mas'ud (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "The blood of a Muslim is inviolable except by one of three: adultery by a married person, a life for a life (homicide), and abandoning Islam and separating from the Islamic body.''
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The blood of the Muslim is inviolable except by one of the three things mentioned in this Hadīth. First, a married person who already had sexual relation with his legitimate wife, yet committed fornication. Second, a Muslim who intentionally kills another Muslim for no legitimate reason. Third, apostasy (a Muslim who abandons Islam and separates from the Islamic body). It is not permissible to shed the blood of a Muslim, except in these three cases and cases that fall under these categories although not mentioned explicitly in this Hadīth. These include homosexuality and incest, both of which fall under the first category; magic, which falls under the third category, and so on.