Musnad Ahmad 30:399

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Abu Salamah, from Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, said: "When one of you wakes up at night, let him not dip his hand into his vessel until he has washed it three times, for he does not know where his hand spent the night."