It is narrated from Sayyiduna Suraqah bin Malik (may Allah be pleased with him) that he asked the Messenger of Allah (peace and blessings be upon him) about a stray animal that comes to his trough: will he have a reward for satisfying it? The Messenger of Allah (peace and blessings be upon him) said: "Yes, there is a reward in giving water to every creature with a thirsting liver."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6570