Musannaf Ibn Abi Shayba 27:425

Abu Mu’awiyah narrated to us, from ‘Asim, from Ibn Sirin, that ‘Uthman was mentioned in his presence. A man said, "They are insulting him." He said, "Woe to them! They insult a man who entered upon the Negus among a group of the Companions of the Messenger of Allah (may Allah bless him and grant him peace), and he gave them all the trial except for him." They said, "And what is the trial that he gave them?" He said, "No one would enter upon him without him gesturing with his head [to bow], but ‘Uthman refused. So he said, 'What prevented you from prostrating as your companions prostrated?' He replied, 'I would not prostrate to anyone other than Allah, the Almighty and Majestic.'"