Musannaf Ibn Abi Shayba 27:405

'Abdullah ibn Idris narrated to us, from Muhammad ibn Ayyub, from Hilal ibn Abi Humayd, who said: 'Abdullah ibn 'Ukaym said: "I will never assist in the killing of a caliph after 'Uthman, ever." It was said to him: "Did you assist in his blood?" He said: "I consider mentioning his misdeeds to be an assistance in his blood."