Musnad Ahmad 1304:14

Abu al-Nadr narrated to us, Abd al-Hamid narrated to us, Shahr ibn Hawshab narrated to me, he said: Asma bint Yazid narrated to me, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Good is tied to the forelocks of horses forever until the Day of Resurrection. Whoever keeps them as preparation in the way of Allah, and spends on them seeking reward in the way of Allah, then their fullness and their hunger, their watering and their thirst, and their dung and their urine, shall be success in his scales on the Day of Resurrection. And whoever keeps them for show, for reputation, for pride, and for arrogance, then their fullness and their hunger, their watering and their thirst, and their dung and their urine, shall be loss in his scales on the Day of Resurrection."