Sahih Muslim | Book 16 | Hadith Number 4154

Narrated by 'Abdullah (b. Mas'ud)
'Abdullah (b. Mas'ud) reported: Allah's Messenger (may peace be upon him) stood up and said: By Him besides Whom there is no god but He, the blood of a Muslim who bears the testimony that there is no god but Allah, and I am His Messenger, may be lawfully shed only in case of three persons: the one who abandons Islam, and deserts the community [Ahmad, one of the narrators, is doubtful whether the Holy Prophet (may peace be upon him) used the word li'l-jama'ah or al-jama'ah), and the married adulterer, and life for life.