Musnad Ahmad 30:2801

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, that the Messenger of Allah, may Allah bless him and grant him peace, said: "If a dog drinks from the vessel of one of you, let him wash it seven times."