Thabit, the freed slave of Abd al-Rahman bin Zayd, reported that he heard Sayyiduna Abu Hurayrah (may Allah be pleased with him) say that the Messenger of Allah (peace and blessings be upon him) said: "When one of you has been asleep, then wakes up and intends to perform ablution, he should not put his hand into the vessel until he pours water over his hand and washes it, for he does not know where his hand spent the night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1207