It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When a dog puts its mouth into a vessel, it should be washed seven times, and the seventh time with dust."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1146