(6) باب مَا يُبَاحُ بِهِ دَمُ الْمُسْلِمِ
'Abdullah (b. Mas'ud) reported Allah's Messenger (ﷺ) as saying:
It is not permissible to take the life of a Muslim who bears testimony (to the fact that there is no god but Allah, and I am the Messenger of Allah, but in one of the three cases: the married adulterer, a life for life, and the deserter of his Din (Islam), abandoning the community.
Reference: Sahih Muslim 1676 a
In-book reference: Book 28, Hadith 34
USC-MSA web (English) reference(deprecated numbering scheme): Book 16, Hadith 4152