(a) It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When a dog laps from the vessel of one of you, it becomes pure by being washed seven times." (b) Sayyiduna Abu Hurayrah (may Allah be pleased with him), Ibn Abbas (may Allah be pleased with them both) and Aishah (may Allah be pleased with her) gave the verdict that a vessel licked by a dog is to be washed seven times.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1142