Musnad Ahmad 1153:542

Bahlul ibn Hakim al-Qarqasani narrated to us, he said: al-Awza'i narrated to us, from al-Zuhri, from 'Urwah, from 'Aishah, she said: "When the Messenger of Allah, peace and blessings of Allah be upon him, intended to sleep, he would perform his ablution for prayer."