Sahih Al Bukhari – Book of Distribution Of Water

Volumn 003, Book 040, Hadith Number 551. 
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Narated By Abu Huraira RA: Allah's Apostle PBUH 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." 

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