Musnad Abu Ya'la al-Mawsili 155:27

Khalid ibn Mirdas narrated to us, Isma'il ibn 'Ayyash narrated to us, from 'Abbad ibn Kathir, from Abu 'Abd Allah, from 'Ata' ibn Yasar, from Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "If one of you is afflicted with judging between the Muslims, then let him not judge while he is angry. And let him treat them equally in looking, in sitting, and in gesturing, and let him not raise his voice against one of the two litigants."