Abu Huraira related that the Prophet (SAW) said: Whenever any of you awakens from his sleep, he should not put his hand into the vessel until he washes it three times, for indeed he does not know where his hand has spent the night.
Sahih Muslim, The book of purity

Al-Nawawi relates in his “Bustan”, from Muhammad ibn Al-Fadl Al-Taymi in his exposition of Sahih Muslim, that one of the heretics [mubtadi’a, innovators], when he heard of this hadith, he said sarcastically: "I know where my hand spent the night – it spent the night in the bed!" And when he arose in the morning, he was found to have stuck his hand in his rear up to his arm.