Musannaf Ibn Abi Shayba 3:1389

Ibn Abi Ghaniyah narrated to us, from his father, from al-Hakam, who said: Recite behind the Imam in that in which he does not recite aloud: in the first two [rak'ahs], the Opening of the Book (al-Fatihah) and a surah; and in the last two, the Opening of the Book (al-Fatihah).