Sahih Bukhari Volume 003, Book 043, Hadith Number 646.
Narated By Abu Huraira : The Prophet said, "A man felt very thirsty while he was on the way, there he came across a well. He went down the well, quenched his thirst and came out. Meanwhile he saw a dog panting and licking mud because of excessive thirst. He said to himself, "This dog is suffering from thirst as I did." So, he went down the well again and filled his shoe with water and watered it. Allah thanked him for that deed and forgave him. The people said, "O Allah's Apostle! Is there a reward for us in serving the animals?" He replied: "Yes, there is a reward for serving any animate (living being)." (See Hadith No. 551)