صحيح مسلم

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.