“Narrated Abdullah: Allah’s Messenger said, “The blood of a Muslim who confesses that none has the right to be worshipped but Allah and that I am His Messenger, cannot be shed except in three cases: in Qisas (equality in punishment) for murder, a married person who commits illegal sexual intercourse and the one who reverts from Islam (Apostate) and leaves the Muslims.”

Bukhari, volume 9, #57

Narrated Ikrima, “Some 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 messenger forbade it, saying, “Do not punish anybody with Allah’s punishment (fire).” I would have killed them according to the statement of Allah’s Messenger, “Whoever changed his Islamic religion, then kill him.”

Bukhari, volume 9, #58

Narrated Abu Bruda, “Abu Musa said…..Behold there was a fettered man beside Abu Musa. Muadh asked, “Who is this (man)?” Abu Musa said, “He was a Jew and became a Muslim and hen reverted back to Judaism.” Then Abu Musa requested Muadh to sit down but Muadh said, “I will not sit down till he has been killed. This is the judgment of Allah and his messenger,” 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