Sahih Muslim Book 16, Hadith Number 4152.

‘Abdullah (b. Mas’ud) reported Allah’s Messenger (may peace be upon him) 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.