It is narrated from Abu Huraira (may Allah be pleased with him) that the Prophet (peace be upon him) said: "When any of you wakes up from sleep, he should wash his hands before putting them into the vessel for ablution, because none of you knows where his hand spent the night."
Hadith Referenceمسند الإمام الشافعي / كتاب الطهارة / 43