Musnad Ahmad 25:1635

Ibn Abi ‘Adi narrated to us, from Sa‘id, from Qatadah, from Musa ibn Salamah, who said: I asked Ibn ‘Abbas about the prayer at al-Batha’ if I missed the prayer in congregation. He said: "Two rak‘ahs; that is the Sunnah of Abu al-Qasim, may Allah bless him and grant him peace."