Allah's Apostle said, "The blood of a Muslim who confesses that none has the right to be worshipped but Allah and that I am His Apostle, cannot be shed except in three cases: In Qisas for murder, a married person who commits illegal sexual intercourse and the one who reverts from Islam (apostate) and leaves the Muslims."
Sahih al-Bukhari, 9:84:57 (link to Book 84)
Some Zanadiqa (atheists) were brought to 'Ali and he burnt them. The news of this event, reached Ibn 'Abbas who said, "If I had been in his place, I would not have burnt them, as Allah's Apostle forbade it, saying, 'Do not punish anybody with Allah's punishment (fire).' I would have killed them according to the statement of Allah's Apostle, 'Whoever changed his Islamic religion, then kill him.'"
Sahih al-Bukhari, 9:84:58
Abu Musa said, "I came to the Prophet along with two men (from the tribe) of Ash'ariyin, one on my right and the other on my left, while Allah's Apostle was brushing his teeth (with a Siwak), and both men asked him for some employment. The Prophet said, 'O Abu Musa (O 'Abdullah bin Qais!).' I said, 'By Him Who sent you with the Truth, these two men did not tell me what was in their hearts and I did not feel (realize) that they were seeking employment.' As if I were looking now at his Siwak being drawn to a corner under his lips, and he said, 'We never (or, we do not) appoint for our affairs anyone who seeks to be employed. But O Abu Musa! (or 'Abdullah bin Qais!) Go to Yemen.'" The Prophet then sent Mu'adh bin Jabal after him and when Mu'adh reached him, he spread out a cushion for him and requested him to get down (and sit on the cushion). Behold: 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 and one of us said, 'I pray and sleep, and I hope that Allah will reward me for my sleep as well as for my prayers.'"
Sahih al-Bukhari, 9:84:64
Whenever I tell you a narration from Allah's Apostle, by Allah, I would rather fall down from the sky than ascribe a false statement to him, but if I tell you something between me and you (not a Hadith) then it was indeed a trick (i.e., I may say things just to cheat my enemy). No doubt I heard Allah's Apostle saying, "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."
Percentage of Muslims in favor of the death penalty for apostates (those who "leave Islam"), by country:
- Turkey 5%
- Lebanon 6%
- Indonesia 30%
- Nigeria 51%
- Pakistan 76%
- Egypt 84%
- Jordan 86%
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