Musnad Ahmad 30:2047

Sulayman narrated to us, Isma'il informed us, 'Utbah ibn Muslim, the freed slave of Banu Taym, informed me, from 'Ubayd ibn Hunayn, the freed slave of Banu Zurayq, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If a dog laps from the vessel of one of you, let him wash it seven times."