Musnad Ahmad 30:3366

Yazid narrated to us, Muhammad—meaning Ibn Ishaq—narrated to us, from Musa ibn Yasar, from Abu Hurairah, and from al-Zuhri, and others, they said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you wakes up, he must not place his hand into the washing vessel until he washes it, for he does not know where his hand spent the night."