It was narrated that Abu Umamah bin Sahl bin Hunaif said. We were with Uthman when he was besieged in the house. He said: Why would they kill me? I heard the Messenger of Allah (ﷺ) say: `It is not permissible (to shed) the blood of a Muslim except in three cases: If a man disbelieves after becoming Muslim, or commits zina after being married or kills someone and is executed in retaliation.`
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ أَبِي أُمَامَةَ بْنِ سَهْلِ بْنِ حُنَيْفٍ، قَالَ كُنَّا مَعَ عُثْمَانَ وَهُوَ مَحْصُورٌ فِي الدَّارِ قَالَ وَلِمَ تَقْتُلُونَنِي سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ لَا يَحِلُّ دَمُ امْرِئٍ مُسْلِمٍ إِلَّا بِإِحْدَى ثَلَاثٍ رَجُلٌ كَفَرَ بَعْدَ إِسْلَامِهِ أَوْ زَنَى بَعْدَ إِحْصَانِهِ أَوْ قَتَلَ نَفْسًا فَيُقْتَلُ بِهَا.
sunnah.com