Narrated Abu Huraira (Radi-Allahu 'anhu):

Allah's Apostle (Sallallahu 'Alaihi Wa Sallam) said, "While a man was
walking he felt thirsty and went down a well and drank water from it. On
coming out of it, he saw a dog panting and eating mud because of excessive
thirst. The man said, 'This (dog) is suffering from the same problem as
that of mine.' So he (went down the well), filled his shoe with water,
caught hold of it with his teeth and climbed up and watered the dog. Allah
thanked him for his (good) deed and forgave him." The people asked, "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."

Bukhari Vol. 3 : No. 551