Musnad Ahmad 1153:1417

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from Abu Ishaq, from al-Aswad, who said: I asked ‘Aishah about the prayer of the Messenger of Allah, may Allah bless him and grant him peace, at night. She said: "He used to sleep during the first part of the night, then he would stand. When it was the time of the pre-dawn (al-sahar), he would perform the Witr prayer, then he would come to his bed. If he had a need, he would approach his wife. Then, when he heard the Adhan, he would spring up. If he was in a state of major ritual impurity (junub), he would pour water over himself; otherwise, he would perform ablution (wudu), then go out to the prayer."