Sahih Bukhari Volume 008, Book 073, Hadith Number 038.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 73. Good Manners and Form (Al-Adab)
Narated By Abu Huraira : Allah's Apostle said, "While a man was walking on a road. he became very thirsty. Then he came across a well, got down into it, drank (of its water) and then came out. Meanwhile he saw a dog panting and licking mud because of excessive thirst. The man said to himself "This dog is suffering from the same state of thirst as I did." So he went down the well (again) and filled his shoe (with water) and held it in his mouth and watered the dog. Allah thanked him for that deed and forgave him." The people asked, "O Allah's Apostle! Is there a reward for us in serving the animals?" He said, "(Yes) There is a reward for serving any animate (living being)."