Musnad Abu Ya'la al-Mawsili 160:45

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