Musnad Abu Ya'la al-Mawsili 155:23

Ibrahim ibn Sa'id al-Jawhari narrated to us, Musa ibn Dawud narrated to us, from Ibn Lahi'ah, from Abu al-Zubayr, Jabir informed me that Abu Hurayrah informed him that the Prophet, peace and blessings of Allah be upon him, said: "If one of you wakes up from his sleep, let him pour over his hands three times before he inserts them [into the vessel], for he does not know where his hand spent the night."