Musannaf Ibn Abi Shayba 3:1346

Abu Mu'awiyah narrated to us, from al-Shaybani, from al-Sha'bi, who said: Umar wrote to Shurayh: "Recite in the first two [rak'ahs] the Opening of the Book (al-Fatiha) and a surah, and in the last two [rak'ahs] the Opening of the Book."