It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "When any of you wakes up from sleep, let him not put his hand into the vessel, but rather wash it three times, for he does not know where his hands spent the night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 206