Musannaf Ibn Abi Shayba 35:191

Narrated to us Abd al-Rahim ibn Sulayman, from Muhammad ibn Amr, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you rises from his sleep, let him pour over his hand from his vessel three times, for he does not know where his hand spent the night."