Musnad Ahmad 1153:1052

Waki’ narrated to us, from Sufyan, from Bard, from ‘Ubadah ibn Nusayy, from Ghudayf ibn al-Harith, from ‘Aishah, who said: "The Prophet, peace and blessings of Allah be upon him, would sometimes perform the Witr prayer before he slept, and sometimes he would perform the Witr prayer after he slept. And he would sometimes perform Ghusl (ritual bath) before he slept, and sometimes he would sleep before performing Ghusl from Janabah (major ritual impurity)."