Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with this same chain (it is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him) that the Messenger of Allah, peace and blessings be upon him, said: "A man was walking on a path when he became extremely thirsty. Then he saw a well, so he went down into it and drank water. When he came out, he saw a dog panting and eating mud out of thirst. The man said: Just as I was extremely thirsty, this dog is also thirsty. So he went down into the well, filled his shoe with water, held it in his mouth, and climbed up, then gave the dog water to drink. Allah appreciated this deed and forgave him." The people said: O Messenger of Allah! Is there a reward for us in (serving) animals as well? He, peace and blessings be upon him, said: "There is a reward for (serving) every living being with a moist liver."