Musnad Ahmad 1153:116

Sufyan narrated to us, Yahya narrated to us, from the nephew of ‘Amrah—meaning Muhammad ibn ‘Abd al-Rahman—from ‘Amrah, from ‘Aishah: "The Prophet, peace and blessings of Allah be upon him, used to lighten the two rak'ahs [of the sunnah prayer] to the extent that I would say: 'Did he recite the Opening of the Book [Al-Fatihah] in them, or not?'"