It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever is given the responsibility of judging between the Muslims, let him treat them equally in seating, in gesture and in look, and let him not raise his voice against one of the two disputing parties more than against the other."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20225