Musannaf Ibn Abi Shayba 13:1971

Abu Bakr narrated to us, he said: Isma'il bin 'Ayyash narrated to us, from Sufyan, from 'Amr bin Ibrahim al-Ansari, from his paternal uncle al-Dahhak, who said: Two men disputed before 'Umar bin al-Khattab and requested his testimony. 'Umar said to them: "If you wish, I will testify and not judge between you, and if you wish, I will judge and not testify."