Musnad Ahmad 30:2866

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that the Messenger of Allah, may Allah bless him and grant him peace, said: "When one of you wakes up from his sleep, let him wash his hand before he puts it into his ablution water, for one of you does not know where his hand spent the night."