Musannaf Ibn Abi Shayba 3:3969

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Hisham bin 'Urwah narrated to us, from his father, from 'Aishah, she said: The Prophet, peace and blessings of Allah be upon him, used to lighten the two rak'ahs of the dawn prayer.