Musannaf Ibn Abi Shayba 35:194

Ibn ‘Ulayyah narrated to us, from Hisham, from Ibn Sirin, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "The purification of the vessel of one of you, when a dog licks it, is that he washes it seven times, the first of them with earth."