The blood of a Muslim man who testifies that there is no god but Allah and that I am the Messenger of Allah should not be lawfully shed but only for one of three reasons: married fornicator, soul for soul, and one who deserts his religion separating himself from the community. (Using translation from Abū Dāʾūd 4352)
قَامَ فِينَا رَسُولُ اللهِ ﷺ فَقَالَ وَالَّذِي لَا إِلَهَ غَيْرُهُ لَا يَحِلُّ دَمُ رَجُلٍ مُسْلِمٍ يَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللهُ وَأَنِّي رَسُولُ اللهِ إِلَّا ثَلَاثَةَ نَفَرٍ التَّارِكُ الْإِسْلَامَ وَالْمُفَارِقُ الْجَمَاعَةَ وَالثَّيِّبُ الزَّانِي وَالنَّفْسُ بِالنَّفْسِ