Musnad Ahmad 1153:216

Yahya narrated to us, from Shu'bah, from Muhammad ibn 'Abd al-Rahman ibn As'ad ibn Zurarah, from 'Amrah, from 'Aishah, who said: "When the dawn would break, the Messenger of Allah (peace and blessings of Allah be upon him) would not pray except two rak'ahs." I (the narrator) say: He recited in them the Opening of the Book (Al-Fatihah).