Musannaf Ibn Abi Shaybah

Hazrat Hilal bin Abi Humaid (may Allah have mercy on him) narrates that Hazrat Abdullah bin Ukeem (may Allah have mercy on him) said: After the martyrdom of Hazrat Uthman, I will never assist in the killing of a caliph. The narrator says that he was asked: Did you assist in his killing? He replied: Certainly, I assisted in his blood to the extent that I used to enumerate his faults.