Sahih Bukhari, Chapter: Jihaad (Fighting for the cause of Allah)

Allah's Messenger said, " Horses are kept for one of three purposes; for some people they are a source of reward, for some others they are a means of shelter (livelyhood) and for some others they are a source of sins. The one for whom they are a source of reward, is he who keeps a horse for Allah's Cause (i.e. Jihad) tying it with a long tether on a meadow or in a garden with the result that whatever it eats from the area of the meadow or the garden where it is tied will be counted as good deeds for his benefit, and if it should break its rope and jump over one or two hillocks then all its dung and its foot marks will be written as good deeds for him; and if it passes by a river and drinks water from it even though he had no intention of watering it, even then he will get the reward for its drinking. As for the man for whom horses are a source of sins, he is the one who keeps a horse for the sake of pride and show-off and showing enmity for Muslims: such a horse will be a source of sins for him. When Allah's Apostle (SAW) was asked about donkeys, he replied, "Nothing has been revealed to me about them except this unique, comprehensive Verse: "'So whoever does a good equal to the weight of an atom (or a small ant) shall see it. And whosever does evil equal to the weight of an atom (or a small ant) shall see it.' " (99:7,8)