Musannaf Ibn Abi Shayba 37:198

Mu'awiyah narrated to us, he said: Sufyan narrated to us, from Ma'mar, from Ibn Tawus, from his father, who said: I said to Ibn Abbas, "Should I forbid my leader from disobedience?" He said, "Do not let it be a tribulation." I said, "But if he commands me to commit an act of disobedience?" He said, "Then at that time [you must refuse]."