It is not permissible to take the life of a Muslim who bears testimony (to the fact that there is no god but Allah, and I am the Messenger of Allah, but in one of the three cases: the married adulterer, a life for life, and the deserter of his Din (Islam), abandoning the community.  

قَالَ رَسُولُ اللَّهِ ﷺ لاَ يَحِلُّ دَمُ امْرِئٍ مُسْلِمٍ يَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنِّي رَسُولُ اللَّهِ إِلاَّ بِإِحْدَى ثَلاَثٍ الثَّيِّبُ الزَّانِ وَالنَّفْسُ بِالنَّفْسِ وَالتَّارِكُ لِدِينِهِ الْمُفَارِقُ لِلْجَمَاعَةِ  

This hadith has been narrated on the authority of A'mash.  

عَنِ الأَعْمَشِ بِهَذَا الإِسْنَادِ مِثْلَهُ  

Messenger of Allah ﷺ stood up and said: By Him besides Whom there is no god but He, the blood of a Muslim who bears the testimony that there is no god but Allah, and I am His Messenger, may be lawfully shed only in case of three persons: the one who abandons Islam, and deserts the community [Ahmad, one of the narrators, is doubtful whether the Prophet ﷺ used the word li'l-jama'ah or al-jama'ah), and the married adulterer, and life for life.  

قَامَ فِينَا رَسُولُ اللَّهِ ﷺ فَقَالَ وَالَّذِي لاَ إِلَهَ غَيْرُهُ لاَ يَحِلُّ دَمُ رَجُلٍ مُسْلِمٍ يَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنِّي رَسُولُ اللَّهِ إِلاَّ ثَلاَثَةُ نَفَرٍ التَّارِكُ الإِسْلاَمَ الْمُفَارِقُ لِلْجَمَاعَةِ أَوِ الْجَمَاعَةَ شَكَّ فِيهِ أَحْمَدُ وَالثَّيِّبُ الزَّانِي وَالنَّفْسُ بِالنَّفْسِ قَالَ الأَعْمَشُ فَحَدَّثْتُ بِهِ إِبْرَاهِيمَ فَحَدَّثَنِي عَنِ الأَسْوَدِ عَنْ عَائِشَةَ بِمِثْلِهِ  

By Him besides Whom there is no god.  

عَنِ الأَعْمَشِ بِالإِسْنَادَيْنِ جَمِيعًا نَحْوَ حَدِيثِ سُفْيَانَ وَلَمْ يَذْكُرَا فِي الْحَدِيثِ قَوْلَهُ وَالَّذِي لاَ إِلَهَ غَيْرُهُ