Musnad Ahmad 1153:1678

Waki' narrated to us, Hisham narrated to us, from his father, from Aisha, who said: "The Messenger of Allah (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."