Musnad Ahmad 512:8

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from 'Urwah ibn al-Zubayr, from Suraqah ibn Malik: That he came to the Messenger of Allah, may Allah bless him and grant him peace, during his illness, and said: "Do you see the stray camel that comes to my camel trough? Is there a reward for me if I give it water?" He said: "Yes, in every thirsty liver, there is a reward."