Musnad Ahmad 284:11

Abu Salamah al-Khuza'i narrated to us, Abd al-Rahman ibn Abi al-Mawali narrated to us, he said: Nafi' ibn Thabit informed me, from Abdullah ibn al-Zubayr, he said: "When the Messenger of Allah, peace and blessings of Allah be upon him, prayed the Isha, he would perform four rak'ahs, and he would perform witr with one prostration. Then he would sleep until he prayed after his prayer at night."