Musannaf Ibn Abi Shayba 36:111

Abu Mu’awiyah narrated to us, from ‘Asim, from Ibn Sirin, that ‘Uthman ibn ‘Affan was mentioned in his presence. A man said, "They insult him." He said, "Woe to them! They insult a man who entered upon the Negus among a group of the companions of Muhammad, may Allah bless him and grant him peace, and he gave them all the *fitnah* except for him." They said, "And what is the *fitnah* that he gave them?" He said, "No one would enter upon him except that he would gesture to him with his head [to bow/prostrate], but ‘Uthman refused. So he said, 'What prevented you from prostrating as your companions prostrated?' He said, 'I would not prostrate to anyone other than Allah.'"