Musnad Ahmad 30:3458

Yazid narrated to us, Hisham informed us, from Muhammad, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "When one of you wakes up from his sleep, let him not dip his hand into his purification water until he pours over it and washes it, for he does not know where his hand spent the night."