Musnad Abu Ya'la al-Mawsili 160:173

Ahmad ibn Isa narrated to us, Abdullah ibn Wahb narrated to us, Talhah ibn Abi Sa’id informed me that Sa’id al-Maqburi narrated to him, from Abu Hurayrah, from the Messenger of Allah (may Allah bless him and grant him peace), who said: "Whoever keeps a horse for the sake of Allah, out of faith and belief in the promise of Allah, then its satiety, its thirst-quenching, its urine, and its dung will be good deeds in his balance on the Day of Resurrection."