Musnad Ahmad 30:1753

Ishaq narrated to us, Malik informed me, from Sumayy, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "While a man was walking on a road, thirst became severe for him. He found a well, so he descended into it and drank, then he came out. Suddenly, there was a dog panting, eating the moist earth out of thirst. The man said: 'This dog has reached the same level of thirst that I had reached.' So he descended into the well and filled his leather sock with water, then held it with his mouth until he climbed out, and he gave the dog to drink. Allah thanked him for it and forgave him." They said: "O Messenger of Allah, is there a reward for us in [serving] animals?" The Messenger of Allah (peace and blessings of Allah be upon him) said: "In every living creature with a moist liver, there is a reward."