Musnad Ahmad 1153:1581

Waki' narrated to us, he said: Hisham ibn 'Urwah narrated to us, from his father, from 'Aishah, she said: The Prophet, peace and blessings of Allah be upon him, used to pray at night while I was lying across between him and the Qiblah. When he intended to perform the Witr, he would wake me up, and I would perform the Witr.