http://www.thereligionofpeace.com/Quran/012-apostasy.htm Qur'an (4:89) - "They but wish that ye should reject Faith, as they do, and thus be on the same footing (as they): But take not friends from their ranks until they flee in the way of Allah (From what is forbidden). But if they turn renegades, seize them and slay them wherever ye find them"
Hadith:
Bukhari (52:260) - "...The Prophet said, 'If somebody (a Muslim) discards his religion, kill him.' "
Bukhari (84:57) - [In the words of] "Allah's Apostle, 'Whoever changed his Islamic religion, then kill him.'"
Bukhari (89:271) - A man who embraces Islam, then reverts to Judaism is to be killed according to "the verdict of Allah and his apostle."
Contemporary Shariah law:
The contemporary (i.e., 1991) Al-Azhar (Cairo) Islamic Research Academy endorsed manual of Islamic Law, Umdat al-Salik (pp. 595-96) states: "Leaving Islam is the ugliest form of unbelief (kufr) and the worst.... When a person who has reached puberty and is sane voluntarily apostasizes from Islam, he deserves to be killed. In such a case, it is obligatory...to ask him to repent and return to Islam. If he does it is accepted from him, but if he refuses, he is immediately killed."