Abu Huraira (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings 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 he does not know where his hands spent the night."
Hadith Referenceمسند الإمام الشافعي / كتاب الطهارة / 44
Hadith Takhrijانظر الحديث الذي قبله برقم (42) (41).