Musannaf Ibn Abi Shayba 37:578

Abu Bakr narrated to us, he said: Abu Usama narrated to us, he said: Isma'il narrated to us, from Qays, who said: When Mu'awiyah and 'Amr arrived in Kufa, Al-Harith ibn al-Azma' came to 'Amr. 'Amr went out while he was riding. Al-Harith said to him: "I came regarding a matter that, had I found you on stable ground, I would have asked you about." 'Amr said: "There is nothing you would have asked me about while I was on stable ground except that I would inform you of it now." He said: "Then inform me about 'Ali and 'Uthman." He said: "Displeasure and favoritism gathered, and the displeasure overcame the favoritism." Then he departed.