Musannaf Ibn Abi Shayba 3:1354

Ibn ‘Ulayyah narrated to us, from Yahya ibn Abi Ishaq, who said: ‘Umar ibn Abi Suhaym narrated to me, saying: ‘Abdullah ibn Mughaffal used to command regarding the prayer in which the Imam does not recite aloud, that one should recite in the prayer, in the first two rak’ahs, the Opening of the Book (Al-Fatihah) and a surah. And in the other two, the Opening of the Book (Al-Fatihah).