Musnad Ahmad 30:2017

Hawdha narrated to us, 'Awf narrated to us, from Muhammad, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, who said: "If one of you wakes up from his sleep and intends to perform ablution, let him not place his hand into the vessel until he washes it, for he does not know where his hand spent the night."