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 one of you gets up from sleep (and) stands to perform ablution, let him pour water over his hands, for he does not know where his hand spent the night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 576