Quran (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."
Quran (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."
Hadith and Sira
Bukhari (52:260) - "...The Prophet said, 'If somebody (a Muslim) discards his religion, kill him.' "
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.'"
Bukhari (11:626) - "The Prophet said, 'No prayer is harder for the hypocrites than the Fajr and the 'Isha' prayers and if they knew the reward for these prayers at their respective times, they would certainly present themselves (in the mosques) even if they had to crawl.' The Prophet added, 'Certainly I decided to order the Mu'adh-dhin (call-maker) to pronounce Iqama and order a man to lead the prayer and then take a fire flame to burn all those who had not left their houses so far for the prayer along with their houses'."
Abu Dawud (4346) - "Was not there a wise man among you who would stand up to him when he saw that I had withheld my hand from accepting his allegiance, and kill him?" Muhammad is chastising his companions for allowing an apostate to "repent" under duress. (The person in question was Muhammad's former scribe, who left him after doubting the authenticity of divine "revelations" - upon finding out that grammatical changes could be made. He was brought back to Muhammad after having been captured in Medina).
al-Muwatta of Imam Malik (36.18.15) - "The Messenger of Allah said, "If someone changes his religion - then strike off his head."
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)