From Sahih Muslim (Hadith 4377):
Abdullah (b. Mas'ud) reported:Allah's Messenger (ﷺ) 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 Prophet (ﷺ) used the word li'l-jama'ah or al-jama'ah), and the married adulterer, and life for life
Reference: Muslim 4377