Musnad Ahmad 25:1260

Muhammad ibn Ja'far narrated to us, Shu'bah and Hajjaj narrated to us, he said: Shu'bah narrated to me, from Qatadah, from Musa ibn Salamah, who said: I asked Ibn Abbas: "How should I pray when I am in Makkah, if I do not pray with the Imam?" He said: "Two rak'ahs, the Sunnah of Abu al-Qasim, may Allah bless him and grant him peace."