Musnad Ahmad 1304:33

Waki’ narrated to us, ‘Abd al-Hamid ibn Bahram narrated to us, from Shahr ibn Hawshab, from Asma’ bint Yazid, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever keeps a horse for the sake of Allah, and spends on it seeking reward, then its fullness, its hunger, its quenching, its thirst, its urine, and its dung will be in his scale on the Day of Resurrection. And whoever keeps a horse for show and for reputation, that will be a loss in his scale on the Day of Resurrection."