It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When any of you wakes up from sleep, let him not dip his hands into the vessel, but rather wash them three times, for he does not know where his hands spent the night."
Sa'id bin al-Musayyib (may Allah have mercy on him) also reported a narration from Sayyiduna Abu Hurayrah (may Allah be pleased with him) with the same meaning.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 202