Musnad Ahmad 30:556

'Abd al-Razzaq and Ibn Bakr narrated to us, Ibn Jurayj informed us, Ziyad informed me, that Thabit, the freed slave of 'Abd al-Rahman ibn Zayd—and Ibn Bakr said: He informed him that he heard Abu Hurayrah saying: The Messenger of Allah, peace and blessings of Allah be upon him, said: "When one of you is sleeping, then he wakes up and intends to perform ablution, let him not place his hand into the vessel until he pours over his hand, for he does not know where his hand spent the night."