Musnad Ahmad 30:1745

Ibrahim narrated to us, Ibn al-Mubarak narrated to us, from Talha ibn Abi Sa'id, I heard Sa'id al-Maqburi narrating that he heard Abu Hurairah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever keeps a horse for the sake of Allah, out of faith in Allah and belief in His promise, then its satiety, its thirst-quenching, its urine, and its dung will be good deeds in his scale on the Day of Resurrection."