Musnad Ahmad 3:54

Ishaq ibn Sulayman narrated to us, he said: I heard Mughira ibn Muslim Abu Salama mentioning from Matar, from Nafi’, from Ibn Umar: That Uthman looked down upon his companions while he was besieged, and he said: "For what do you kill me? For I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'The blood of a Muslim man is not lawful except for one of three: a man who committed adultery after having been chaste, so upon him is stoning; or one who killed intentionally, so upon him is retaliation (qisas); or one who apostatized after his Islam, so upon him is killing.'" By Allah, I have not committed adultery in the pre-Islamic period nor in Islam, nor have I killed anyone so that I should offer myself for retaliation, nor have I apostatized since I accepted Islam. I bear witness that there is no god but Allah, and that Muhammad is His servant and His messenger.