Musnad Ahmad 30:3211

Muhammad ibn Ja'far narrated to us, he said: And he was asked about the vessel from which a dog has lapped. He said: Sa'id narrated to us, from Ayyub, from Ibn Sirin, from Abu Hurayrah, from the Messenger of Allah (peace and blessings of Allah be upon him) that he said: "It shall be washed seven times, the first of them with earth."