The Book of Greetings
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا أَبُو خَالِدٍ الأَحْمَرُ، عَنْ هِشَامٍ، عَنْ مُحَمَّدٍ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم " أَنَّ امْرَأَةً بَغِيًّا رَأَتْ كَلْبًا فِي يَوْمٍ حَارٍّ يُطِيفُ بِبِئْرٍ قَدْ أَدْلَعَ لِسَانَهُ مِنَ الْعَطَشِ فَنَزَعَتْ لَهُ بِمُوقِهَا فَغُفِرَ لَهَا " .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)
From Sahih Muslim, whose reports the scholars accept as authentic.