Musnad Ahmad 512:7

Ya'qub narrated to us, my father narrated to us, from Salih, and Ibn Shihab narrated that 'Abd al-Rahman ibn Malik informed him that Suraqah ibn Ju'sham entered upon the Messenger of Allah, may Allah bless him and grant him peace, during his illness in which he passed away. He said: So I began to ask the Messenger of Allah, may Allah bless him and grant him peace, until I could not remember what I was asking him about. He said: "Mention it." He said: And among what I asked him about was that I said: "O Messenger of Allah, a stray animal comes to my watering troughs, and I have filled them with water for my camels; do I have a reward if I give it to drink?" The Messenger of Allah, may Allah bless him and grant him peace, said: "Yes, in the watering of every thirsty liver, there is a reward for the sake of Allah."