It is narrated from Sayyiduna Abdullah bin Mughaffal (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: "When a dog puts its mouth into a vessel, wash that vessel seven times, and rub it with earth the eighth time." (This means to rub it with earth once, and the number is not intended.)
The dog is impure (najis), and its saliva is also impure and filled with germs, as modern science has acknowledged. The Wise Lawgiver (Shari‘ Hakim) has commanded, for the sake of hygiene, that if a dog puts its mouth into a vessel, that vessel should be washed seven times with water and once scrubbed with earth. In the authentic hadith it is stated: «اولاهن بالتراب» the first time should be with earth.
Some people consider washing the vessel three times to be sufficient, which is completely contrary to the hadith of the Messenger (sallallahu alayhi wa sallam).
Furthermore, this ruling includes all types of dogs, whether they are domesticated, guard dogs, hunting dogs, or of any breed; their leftover water is impure. It is not permissible to drink this water or to perform ablution (wudu) with it.
From all the preceding ahadith, it is understood that: the water of the sea is pure, likewise, water in large quantities such as that of a well or pond, etc., is also pure. Water left over after a woman’s use is also pure. Water left over by a cat is also pure, and it is permissible to perform ablution (wudu) with it. However, if the water is little, or its characteristics—color, smell, or taste—have changed, or a dog has put its mouth into it, then that water is impure, and it is not permissible to perform ablution (wudu) with it.