Sahih Al Bukhari – Book of Virtues And Merits Of The Prophet (pbuh) And His Companions

Volumn 004, Book 056, Hadith Number 673.
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Narated By Abu Huraira : The Prophet said, "While a dog was going round a well and was about to die of thirst, an Israeli prostitute saw it and took off her shoe and watered it. So Allah forgave her because of that good deed."