Musannaf Ibn Abi Shayba 37:161

Ghundar narrated to us, from Shu'bah, from Ali ibn Mudrik, who said: I heard Abu Salih say: A man from the companions of the Prophet, may Allah bless him and grant him peace, stood up among us on the day of al-Jur'ah. He said: Uthman ibn Affan had sent Sa'id ibn al-'As to Kufa. He said: So the people of Kufa went out and caught up with him. He said: A man from the people said, "I am upon the Sunnah." He replied: "You are not upon the Sunnah until the shepherd is compassionate and the subjects are sincere."