Musnad Abu Ya'la al-Mawsili 209:46

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 tested with judging between the Muslims, let him not judge while he is angry. And let him make them equal in his gaze, his seating, and his gesturing. And let him not raise his voice at one of the two litigants above the other."