Sahih Muslim
The Book Pertaining to the Oath, for Establishing the Responsibility of Murders, Fighting, Requital and Blood-Wit (Kitab Al-Kitab Al-Qasama wal-Muharaba wal-Qisas wal-Diyat)
Book Number: 016 / 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.