Musannaf Ibn Abi Shayba 3:4002

Abu Bakr narrated to us, he said: Ibn 'Ulayyah narrated to us, from 'Abd al-Rahman ibn Ishaq, from al-Zuhri, from 'Urwah, from 'Aishah, that the Prophet, peace and blessings of Allah be upon him, used to lie down when he had prayed the two prostrations (rak'ahs) before the dawn.