Musnad Ahmad 830:21

Sufyan narrated to us, from ‘Abd al-Malik ibn ‘Umayr, from ‘Abd al-Rahman ibn Abi Bakrah, from his father, from the Prophet (peace and blessings of Allah be upon him), who said: "It is not appropriate for a judge"—and Sufyan said once: "for a ruler"—"to judge between two people while he is angry."