Musannaf Ibn Abi Shayba 3:1345

Abu Bakr narrated to us, he said: Isma'il ibn 'Ulayyah narrated to us, from Ayyub, from Ibn Sirin, who said: I was informed that Ibn Mas'ud used to recite in the Dhuhr and 'Asr prayers, in the first two rak'ahs, the Opening of the Book (Al-Fatiha) and whatever was easy [of the Quran]. And in the last two [rak'ahs], [he would recite] the Opening of the Book.