Musannaf Ibn Abi Shayba 26:184

'Abd al-A'la narrated to us, from Yunus, from al-Hasan, that 'Umar said: "Whoever has learned 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 it." He asked: "What is it?" He replied: "One-sixth." He asked: "With whom?" He replied: "I do not know." He said: "May you not know! What use are you then?"