Musnad Ahmad 30:3490

Ishaq narrated to us, Awf informed us, from Anas ibn Sirin, who said—and Awf said: I do not know it except from Abu Hurairah—who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "A prostitute woman was forgiven. She passed by a dog at the top of a well, panting, and thirst had nearly killed it. So she took off her leather sock, tied it with her head covering, and drew water for it. Thus, she was forgiven for that."