Musnad Ahmad 132:33

Abu al-Walid al-Tayalisi narrated to us, he said: Layth narrated to us, from Yazid ibn Abi Habib, from Ibn Mu'adh ibn Anas, from his father—and he had companionship [with the Prophet]—he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Ride these animals while they are healthy, and leave them [to rest] while they are healthy, and do not take them as chairs."