Abu Huraira reported: I entered upon Uthman, may Allah be pleased with him, on the day he was under siege in his house, and I said, “O leader of the believers, I have come to give support or fight.” Uthman said, “O Abu Huraira, would it please you to ‘kill the people altogether’ (5:32) including me?” I said no. Uthman said, “By Allah, if you have killed one man, it is as if you have killed all the people.” So I returned and I did not fight.
Source: al-Ṭabaqāt al-Kubrá 2871
Grade: Sahih (authentic) according to Ahmad Shakir
عَنْ أَبِي هُرَيْرَةَ قَالَ دَخَلْتُ عَلَى عُثْمَانَ رضي الله عنه يَوْمَ الدَّارِ فَقُلْتُ يَا أَمِيرَ الْمُؤْمِنِينَ طَابٌ أَمْ ضَرْبٌ فَقَالَ لِي يَا أَبَا هُرَيْرَةَ أَيَسُرُّكَ أَنْ تَقْتُلَ النَّاسَ جَمِيعًا وَإِيَّايَ مَعَهُمْ فَقُلْتُ لا فَقَالَ وَاللَّهِ لَئِنْ قَتَلْتَ رَجُلا وَاحِدًا لَكَأَنَّمَا قَتَلْتَ النَّاسَ جَمِيعًا فَرَجَعْتُ فَلَمْ أُقَاتِلْ
2871 الطبقات الكبرى لابن سعد
1/666 المحدث أحمد شاكر خلاصة حكم المحدث إسناده صحيح جدا في عمدة التفسير