Wusti - Right from their holy books:
The Koran
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."
From the Hadith:
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.'"
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."