It was narrated that 'Uthman bin 'Affan said:"I heard the Messenger of Allah [SAW] say: 'It is not permissible to shed the blood of a Muslim except in three cases: A man who commits adultery after having married; or one who kills another person, who is to be killed; or who reverts to Kufr after having accepted Islam, who is to be killed.'"
أَخْبَرَنَا مُؤَمَّلُ بْنُ إِهَابٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، قَالَ أَخْبَرَنِي ابْنُ جُرَيْجٍ، عَنْ أَبِي النَّضْرِ، عَنْ بُسْرِ بْنِ سَعِيدٍ، عَنْ عُثْمَانَ بْنِ عَفَّانَ، قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " لاَ يَحِلُّ دَمُ امْرِئٍ مُسْلِمٍ إِلاَّ بِثَلاَثٍ أَنْ يَزْنِيَ بَعْدَ مَا أُحْصِنَ أَوْ يَقْتُلَ إِنْسَانًا فَيُقْتَلُ أَوْ يَكْفُرَ بَعْدَ إِسْلاَمِهِ فَيُقْتَلُ " .
Nesai
The Book of Fighting [The Prohibition of Bloodshed] • Hadith 4058