Al-Muwatta Hadith 21.3

Yahya related to me from Malik from Zayd ibn Aslam from Abu Salih as-Samman from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "Horses are a reward for one man, a protection for another, a burden for another. The one who has them as a reward is the one who dedicates them for use in the way of Allah, and tethers them in a meadow or grassland. Whatever the horse enjoys of the grassland or meadow in the length of its tether are good deeds for him. If it breaks its tether and goes over a hillock or two, its tracks and droppings are good deeds for him. If it crosses a river and drinks from it while he did not mean to allow it to drink it, that counts as good deeds for him, and the horse is a reward for him. Another man uses his horse to gain self reliance and up-standingness and does not forget Allah's right on their necks and backs (i.e. he does not ill treat or over-work them). Horses are a protection for him . Another man uses them out of pride to show them off and in hostility to the people of Islam. They are a burden on that man." The Messenger of Allah, may Allah bless him and grant him peace, was asked about donkeys, and he said, "Nothing has been revealed to me about them except this single all-inclusive ayat, 'Whoever does an atom of good will see it, and whoever does an atom of evil, will see it.' " (Sura 99 Ayats 7,8) .