Sahih Bukhari, Chapter: Ablutions (Wudu')

The Prophet said, "A man saw a dog eating mud from (the severity of) thirst. So, that man took a shoe (and filled it) with water and kept on pouring the water for the dog till it quenched its thirst. So Allah approved of his deed and made him to enter Paradise." And narrated Hamza bin 'Abdullah: My father said. "During the lifetime of Allah's Apostle, the dogs used to urinate, and pass through the mosques (come and go), nevertheless they never used to sprinkle water on it (urine of the dog.)"