Musnad Ahmad 30:486

'Abd al-Razzaq narrated to us, Hisham bin Hassan narrated to us, from Ibn Sirin, from Abu Hurairah, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said. And he said: Ma'mar narrated to us, from Ayyub, from Ibn Sirin, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, he said: "If a dog laps from a vessel, then wash it seven times."