(a) Dawud bin Abi Hind (may Allah have mercy on him) says: I heard Sa'id bin al-Musayyib (may Allah have mercy on him) say that all water is pure; nothing makes it impure.
(b) It is also reported from Sa'id bin al-Musayyib (may Allah have mercy on him): We asked him about the pools from which dogs drink. He said: Water was sent down pure, and nothing makes it impure.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1224