Musnad Ahmad 1153:2150

'Abdullah ibn Yazid narrated to us, he said: Sa'id—meaning Ibn Abi Ayyub—narrated to us, Abu al-Aswad narrated to us, from 'Urwah, from 'Aishah, she said: "When the Messenger of Allah (peace and blessings of Allah be upon him) prayed the two rak'ahs of the dawn (prayer), he would lie down on his right side."