Musnad Ahmad 30:554

'Abd al-Razzaq narrated to us, Ibn Jurayj informed us, Ziyad—meaning Ibn Sa'd—informed me, that Thabit ibn 'Iyad, the freed slave of 'Abd al-Rahman ibn Zayd, informed him that he heard Abu Hurayrah say: 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."