Musnad Ahmad 830:94

'Abd al-Rahman ibn Muhammad al-Muharibi narrated to us, 'Abd al-Malik ibn 'Umayr narrated to us, Ibn Abi Bakrah narrated to me, that his father ordered him to write to a son of his who was a judge in Sijistan: "To proceed: Do not judge between two people while you are angry, for I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'No one should judge between two people while he is angry.'"