Musnad Ahmad 30:2115

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