(a) It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him): "When a dog puts its mouth into a vessel, wash it seven times, and the first time (it should be washed) with dust."
(b) Sayyiduna Abdullah bin Mughaffal (may Allah be pleased with him) also mentions dust from the Prophet (peace and blessings be upon him).
This hadith is gharib (unusual); if Mu'adh preserved it accurately, then it is hasan (good), because the mention of dust in this hadith was not narrated by any trustworthy narrator other than Ibn Sirin from Abu Hurayrah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1148