Musnad Ahmad 1153:762

Aswad ibn Amir narrated to us, he said: Isra'il narrated to us, from Abu Ishaq, from al-Aswad, from 'Aishah, she said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to sleep in the first part of the night and stand (in prayer) in the last part of it."