Musannaf Ibn Abi Shayba 3:1253

Ibn ‘Ulayyah narrated to us, from Ayyub, from Abu al-‘Aliyah al-Barra’, who said: I said to Ibn ‘Umar, "Should I recite in every rak’ah?" He replied, "I would be ashamed before the Lord of this House not to recite the Opening of the Book (al-Fatihah) and whatever is easy [to recite] in every rak’ah." I asked Ibn ‘Abbas, and he said, "It is up to you; if you wish, recite less of it, and if you wish, recite more."