The Qur’an:
Qur’an (4:89) – “They wish that you should disbelieve as they disbelieve, and then you would be equal; therefore take not to yourselves friends of them, until they emigrate in the way of God; then, if they turn their backs, take them, and slay them wherever you find them; take not to yourselves any one of them as friend or helper.”Qur’an (9:11-12) – “But if they repent and establish worship and pay the poor-due, then are they your brethren in religion. We detail Our revelations for a people who have knowledge. And if they break their pledges after their treaty (hath been made with you) and assail your religion, then fight the heads of disbelief – Lo! they have no binding oaths – in order that they may desist.”Other verses that seem to support the many Hadith demanding death for apostates are Qur’an verses 2:217, 9:73-74, 88:21, 5:54, and 9:66.From the Hadith:

The reason why executing apostates has always been well-ensconced in Islamic law is that there is an indisputable record of Muhammad and his companions doing exactly that according to the reliable Hadith.  According to verse 4:80 of the Quran: “Whoso obeyeth the Messenger obeyeth Allah.”

Bukhari (52:260) – “…The Prophet said, ‘If somebody (a Muslim) discards his religion, kill him.’ ”  Note that there is no distinction as to how that Muslim came to be a Muslim.

Bukhari (83:37) – “Allah’s Apostle never killed anyone except in one of the following three situations: (1) A person who killed somebody unjustly, was killed (in Qisas,) (2) a married person who committed illegal sexual intercourse and (3) a man who fought against Allah and His Apostle and deserted Islam and became an apostate.”

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.”

Bukhari (84:58) – “There was a fettered man beside Abu Muisa. Mu’adh asked, ‘Who is this (man)?’  Abu Muisa said, ‘He was a Jew and became a Muslim and then reverted back to Judaism.’  Then Abu Muisa requested Mu’adh to sit down but Mu’adh said, ‘I will not sit down till he has been killed. This is the judgment of Allah and His Apostle (for such cases) and repeated it thrice.’  Then Abu Musa ordered that the man be killed, and he was killed. Abu Musa added, ‘Then we discussed the night prayers'”

Bukhari (84:64-65) – “Allah’s Apostle: ‘During the last days there will appear some young foolish people who will say the best words but their faith will not go beyond their throats (i.e. they will have no faith) and will go out from (leave) their religion as an arrow goes out of the game. So, wherever you find them, kill them, for whoever kills them shall have reward on the Day of Resurrection.'”

Reliance of the Traveller (Islamic Law) o8.1 – “When a person who has reached puberty and is sane voluntarily apostatizes from Islam, he deserves to be killed.”  (o8.4 affirms that there is no penalty for killing an apostate).

Islamic Law:

There is also a consensus by all four schools of Sunni Islamic jurisprudence (i.e., Maliki, Hanbali, Hanafi, and Shafii), as well as classical Shiite jurists, that apostates from Islam must be put to death.  The process of declaring a person to be an apostate is known as takfir and the disbeliever is called a murtad.

Averroes (d. 1198), the renowned philosopher and scholar of the natural sciences, who was also an important Maliki jurist, provided this typical Muslim legal opinion on the punishment for apostasy: “An apostate…is to be executed by agreement in the case of a man, because of the words of the Prophet, ‘Slay those who change their din [religion]’…Asking the apostate to repent was stipulated as a condition…prior to his execution.”

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.”

Immediately after Muhammad’s death, several tribes wanted to leave Islam and return to their preferred religion.  In a conflict known as the Riddah (apostasy) Wars, they were slaughtered in such places remembered as “Garden of Death” and “Gulley of Blood” during the first caliph Abu Bakr’s aggressive and violent campaign to force submission (and keep the tribute payments flowing back to Mecca, of course).  Within months, a great many people were dead, including Muslims who had memorized the Qur’an by heart.

As Abu Bakr, Muhammad’s closest companion, explained in a letter at the time, his prophet “struck whoever turned his back to Him until he came to Islam, willingly or grudgingly.”  Thus did Abu Bakr promise to “burn them with fire, slaughter them by any means, and take women and children captive” any who left Islam. (al-Tabari v10 p.55-57)

Ali, the fourth “Rightly Guided Caliph” was Muhammad’s son-in-law and one of the first converts to Islam.  He also had people burned alive for wanting to follow their conscience.  An old man named Rumahis b. Mansur, who regretted leaving Christianity and vowed not to remain a Muslim, was beheaded by Ali.  (al-Tabari v.17 p.191).

… defenders usually quote verse 2:256 to Western audiences.  The verse states “Let there be no compulsion in religion, for truth stands out from error.”  They may also include a fragment of verse 10:99-100, “Wouldst thou (Muhammad) compel men until they are believers?”  What they don’t say is that Muslim scholars agree that both verses were spoken by Muhammad during an earlier time in his teachings, when he did not have the power to compel others.  They are abrogated by later verses, such as verse 9:29, which clearly commands Muslims to fight unbelievers until they relent and either accept Islam or a state of humiliation under Islamic rule (an obvious illustration of compulsion).

These apologists also ignore the actions of Muhammad at Mecca and those of his companions following his death, particularly the bloody Ridda Wars.  How could those closest to him have felt that there should be “no compulsion in religion” if they were instructed to kill anyone who wanted to leave Islam? How could the mandated killing of apostates have become a part of Islamic law?

One of the world’s most respected Sunni scholars, Yusuf al-Qaradawi, admitted in 2012 that if Muslims had “gotten rid of the apostasy punishment, Islam wouldn’t exist today”.  (Astonishingly enough, he was not apologizing for the beheading, torture, burning and murder of millions but rather trying to justify it). Video Karadawi – Apostasy

And, while some apologists bend the truth in order to distance Islam from one of its most draconian rules, one of the world’s most popular [Zakir Naik] recently affirmed that the death penalty should be applied to those who leave Islam and share their faith with others.  (Ironically Zakir Naik made his comments on a British television channel called Peace TV).