Musannaf Ibn Abi Shayba 3:1355

Hafs narrated to us, from Ash'ath, from Ibn Sirin, who said: They used to say: Recite in the first two [rak'ahs] with the Opening of the Book (Al-Fatiha) and a surah. And in the last [rak'ah], with the Opening of the Book (Al-Fatiha).