Musnad Ahmad 25:25

Muhammad ibn Abd al-Rahman al-Tafawi narrated to us, Ayyub narrated to us, from Qatadah, from Musa ibn Salamah, who said: We were with Ibn Abbas in Makkah, and I said: "When we are with you, we pray four [rak'ahs], but when we return to our dwellings, we pray two." He said: "That is the Sunnah of Abu al-Qasim, may Allah bless him and grant him peace."