Sayyidah Umm Salamah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), narrates that the Prophet (peace and blessings be upon him) said: "When one of you is put to the trial of being a judge between the Muslims, he must never give judgment while he is angry; he should treat them equally in looking at them, in their seating, and in gestures; and he should not raise his voice against one of the two parties more than against the other."
Abu Ya'la recorded it, and al-Tabarani in al-Mu'jam al-Kabir in abridged form; its chain contains Abbad bin Kathir al-Thaqafi, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأحكام / 7017