Musnad Ahmad 1153:61

'Abd al-Rahman ibn Mahdi narrated to us, from Malik, from al-Zuhri, from 'Urwah, from 'Aishah, "That the Prophet, peace and blessings of Allah be upon him, used to pray eleven rak'ahs at night, performing witr with one of them. When he finished his prayer, he would lie down on his right side."