Musnad Ahmad 830:149

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Abd al-Malik ibn ‘Umayr, from ‘Abd al-Rahman ibn Abi Bakrah, who said: Abu Bakrah wrote to his son while he was a governor in Sijistan: "Do not judge between two men while you are angry, for I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'A judge shall not judge between two people, or two disputants, while he is angry.'"