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Abu Hurayrah RA said: The Messenger of Allaah PBUH said: “The purification of the vessel of one of you, if a dog licks it, is to wash it seven times, the first time with soil.” [Sahih Muslim 279]
‘Abd-Allaah ibn al-Mughaffal said: The Messenger of Allaah PBUH said: “If a dog licks the vessel of one of you, let him wash it seven times and rub it with soil the eighth time.” [Sahih Muslim 280]
These hadith have to do with a vessel that a person would eat or drink from afterwards. To say that the saliva of dogs is najis and not just something that is harmful for humans to indirectly injest is subject to different interpretations.