Sahih Bukhari Volume 4, Book 54, Hadith Number 538.

Narated By Abu Huraira : Allah’s Apostle said, “A prostitute was forgiven by Allah, because, passing by a panting dog near a well and seeing that the dog was about to die of thirst, she took off her shoe, and tying it with her head-cover she drew out some water for it. So, Allah forgave her because of that.”