Musnad Ahmad 1153:208

Isma'il narrated to us, he said: 'Abd al-Rahman ibn Ishaq informed us, from al-Zuhri, from 'Urwah, from 'Aishah, who said: "When the Messenger of Allah, peace and blessings of Allah be upon him, would pray the two rak'ahs of the dawn (prayer), he would lie down on his right side."