It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever's religion turns against the religion of the Muslims, strike his neck." And he said: "When he bears witness that there is no god worthy of worship except Allah and that Muhammad (peace and blessings be upon him) is the Messenger of Allah, then there is no way against him, unless he commits something for which the prescribed punishment (hadd) is established upon him."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11567