Musnad Ahmad 30:3568

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