Musnad Ahmad 25:778

'Affan narrated to us, Hammam narrated to us, he said: Qatadah informed us, from Musa ibn Salamah: That he asked Ibn 'Abbas about the prayer at al-Batha' if one does not catch the prayer with the Imam? He said: "Two rak'ahs, the Sunnah of Abu al-Qasim, may Allah bless him and grant him peace."