Sahih Muslim 2244
Abu Huraira reported Allah's Messenger (ﷺ) as sayings:
A person suffered from intense thirst while on a journey, when he found a well. He climbed down into it and drank (water) and then came out and saw a dog lolling its tongue on account of thirst and eating the moistened earth. The person said: This dog has suffered from thirst as I had suffered from it. He climbed down into the well, filled his shoe with water, then caught it in his mouth until he climbed up and made the dog drink it. So Allah appreciated this act of his and pardoned him. Then (the Companions around him) said: Allah's Messenger, is there for us a reward even for (serving) such animals? He said: Yes, there is a reward for service to every living animal.
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكِ بْنِ أَنَسٍ، فِيمَا قُرِئَ عَلَيْهِ عَنْ سُمَىٍّ، مَوْلَى أَبِي
 بَكْرٍ عَنْ أَبِي صَالِحٍ السَّمَّانِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏
 بَيْنَمَا رَجُلٌ يَمْشِي بِطَرِيقٍ اشْتَدَّ عَلَيْهِ الْعَطَشُ فَوَجَدَ بِئْرًا فَنَزَلَ فِيهَا فَشَرِبَ ثُمَّ خَرَجَ فَإِذَا
 كَلْبٌ يَلْهَثُ يَأْكُلُ الثَّرَى مِنَ الْعَطَشِ فَقَالَ الرَّجُلُ لَقَدْ بَلَغَ هَذَا الْكَلْبَ مِنَ الْعَطَشِ مِثْلُ الَّذِي
 كَانَ بَلَغَ مِنِّي ‏.‏ فَنَزَلَ الْبِئْرَ فَمَلأَ خُفَّهُ مَاءً ثُمَّ أَمْسَكَهُ بِفِيهِ حَتَّى رَقِيَ فَسَقَى الْكَلْبَ فَشَكَرَ
 اللَّهُ لَهُ فَغَفَرَ لَهُ ‏"‏ ‏.‏ قَالُوا يَا رَسُولَ اللَّهِ وَإِنَّ لَنَا فِي هَذِهِ الْبَهَائِمِ لأَجْرًا فَقَالَ ‏"‏ فِي كُلِّ
 كَبِدٍ رَطْبَةٍ أَجْرٌ ‏"‏ ‏.‏