Musnad Ahmad 28:596

Harun ibn Ma'ruf narrated to us, Abdullah ibn Wahb narrated to us, Usamah informed me, that 'Amr ibn Shu'ayb narrated to him, from his father, from his grandfather, that a man came to the Messenger of Allah, peace and blessings of Allah be upon him, and said: "I draw water into my cistern until I have filled it for my family, then a camel belonging to someone else comes to me and I give it to drink. Is there any reward for me in that?" The Messenger of Allah, peace and blessings of Allah be upon him, said: "In every living being with a burning liver, there is a reward."