Musannaf Ibn Abi Shayba 20:47

Abu Bakr narrated to us, he said: Abd al-A’la ibn Abd al-A’la narrated to us, from Yunus, from al-Hasan, that Umar said: "Who knows the judgment of the Messenger of Allah, may Allah bless him and grant him peace, regarding the grandfather?" Ma’qil ibn Yasar al-Muzani said: "Regarding us, the Messenger of Allah, may Allah bless him and grant him peace, judged with it." He (Umar) said: "With what?" He said: "With one-sixth." He said: "With whom?" He said: "I do not know." He said: "May you not know! What use are you then?"