حَدَّثَنَا أَحْمَدُ بْنُ عَبْدَةَ، أَنْبَأَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ أَبِي أُمَامَةَ بْنِ سَهْلِ بْنِ حُنَيْفٍ، أَنَّ عُثْمَانَ بْنَ عَفَّانَ، أَشْرَفَ عَلَيْهِمْ فَسَمِعَهُمْ وَهُمْ، يَذْكُرُونَ الْقَتْلَ فَقَالَ إِنَّهُمْ لَيَتَوَاعَدُونِي بِالْقَتْلِ فَلِمَ يَقْتُلُونِي وَقَدْ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ لا يَحِلُّ دَمُ امْرِئٍ مُسْلِمٍ إِلاَّ فِي إِحْدَى ثَلاَثٍ رَجُلٌ زَنَى وَهُوَ مُحْصَنٌ فَرُجِمَ أَوْ رَجُلٌ قَتَلَ نَفْسًا بِغَيْرِ نَفْسٍ أَوْ رَجُلٌ ارْتَدَّ بَعْدَ إِسْلاَمِهِ ‏"‏ ‏.‏ فَوَاللَّهِ مَا زَنَيْتُ فِي جَاهِلِيَّةٍ وَلاَ فِي إِسْلاَمٍ وَلاَ قَتَلْتُ نَفْسًا مُسْلِمَةً وَلاَ ارْتَدَدْتُ مُنْذُ أَسْلَمْتُ ‏.‏

It was narrated from Abu Umamah bin Sahl bin Hunaif that: `Uthman bin 'Affan looked at them when they spoke of killing. He said: “Are they kill threatening to kill me? Why would they kill me? I heard the Messenger of Allah (PBUH) say: “It is not lawful to shed the blood of a Muslim except in one of three (cases): a man who commits adultery when he is a married person, then he should be stoned; a man who kills a soul not in retaliation for murder; and a man who apostatizes after becoming Muslim.' By Allah (SWT), I never committed adultery either during Ignorance days nor in Islam, and I have never killed a Muslim soul, and I have not apostatized since I became Muslim.” Translation Not Available

Book reference: Sunan Ibn Majah Book 20 Hadith 2533
Web reference: Sunan Ibn Majah Volume 3 Book 20 Hadith 2533