Sahih Bukhari
Virtues and Merits of the Prophet (pbuh) and his Companions
Volume: 004 / Book Number: 056 / Hadith Number: 673
Narrated 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."