Musnad Ahmad 1153:1924

Ibn Numayr narrated to us, he said: Hisham narrated to us, from his father, from Aisha, she said: The Messenger of Allah (peace and blessings of Allah be upon him) used to pray while I was lying across between him and the Qibla, on the bedding upon which he and his family would sleep. When he wanted to perform the Witr prayer, he would wake me up, and I would perform the Witr.