Musnad Ahmad 284:6

Khalaf ibn al-Walid narrated to us, he said: Abdullah ibn al-Mubarak narrated to us, he said: Mus’ab ibn Thabit narrated to me, that there was a dispute between Abdullah ibn al-Zubayr and his brother, ‘Amr ibn al-Zubayr. Abdullah ibn al-Zubayr entered upon Sa’id ibn al-‘As, and ‘Amr ibn al-Zubayr was with him on the couch. Sa’id said to Abdullah ibn al-Zubayr: "Come here [sit here]." He said: "No, the decree of the Messenger of Allah (may Allah bless him and grant him peace) — or the Sunnah of the Messenger of Allah (may Allah bless him and grant him peace) — is that the two disputing parties sit before the judge."