Sahih Muslim
The Book on Salutations and Greetings (Kitab As-Salam)
Book Number: 026 / Hadith Number: 5578
Abu Huraira reported Allah's Messenger (may pace be upon him) as saying: A prostitute saw a dog moving around a well on a hot day and hanging out its tongue because of thirst. She drew water for it in her shoe and she was pardoned (for this act of hers).