It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When one of you wakes up from his sleep, he should not put his hand into the water of the vessel until he washes it, for he does not know where his hand spent the night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 575