Musnad Ahmad 1153:887

'Affan narrated to us, he said: Shu'bah narrated to us, Abu al-Mu'ammal said: He informed me, he said: I heard al-Zuhri, from 'Urwah, from 'Aishah, "That the Messenger of Allah, peace and blessings of Allah be upon him, when he prayed two rak'ahs before the dawn, he would sometimes lie down."