Sayyiduna Abu Hurayrah (may Allah be pleased with him) says: I heard the Messenger of Allah (peace and blessings be upon him) saying: "When any of you wakes up from sleep, let him not put his hand into the vessel, but rather wash it three times, for none of you knows where his hand spent the night, or where his hand was roaming."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 208