كتاب السلام
39
The Book of Greetings
(41)
Chapter: The Virtue Of Giving Food And Water To Animals Which Are Unlawful To Eat
(41)
باب فَضْلِ سَاقِي الْبَهَائِمِ الْمُحْتَرَمَةِ وَإِطْعَامِهَا ‏‏

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).
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 2245 a
In-book reference : Book 39, Hadith 211
USC-MSA web (English) reference : Book 26, Hadith 5578
  (deprecated numbering scheme)