(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 drinks from someone's vessel, wash it seven times." (b) It is narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When a dog licks a vessel, it should be washed three, five or seven times." This narration is very weak; Abd al-Wahhab bin al-Dahhak is matruk (abandoned), and Isma'il bin Ayyash is not taken as evidence, especially when he narrates from the people of the Hijaz. (c) In the narration of Abu al-Zinad, washing seven times is mentioned, which is reported by trustworthy narrators.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1141
(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