Musannaf Ibn Abi Shayba 13:3404

Abu Bakr narrated to us, he said: Mu'tamir narrated to us, from his father, he said: A man bore witness before Umar ibn Abd al-Aziz regarding a testimony. The one against whom the testimony was given said: "His testimony is not accepted." He (Umar) said: "And why?" He said: "He does not know who his father is." He (Umar) said: "Bring me a witness other than him."