Sahih al-Bukhari 2277 (Book 42, Hadith 19)

Narrated Abu Huraira: Allah’s Messenger (ﷺ) said, “Keeping horses may be a source of reward to some (man), a shelter to another
(i.e. means of earning one’s living), or a burden to a third. He to whom the horse will be a source of
reward is the one who keeps it in Allah’s Cause (prepare it for holy battles) and ties it by a long rope
in a pasture (or a garden). He will get a reward equal to what its long rope allows it to eat in the
pasture or the garden, and if that horse breaks its rope and crosses one or two hills, then all its
footsteps and its dung will be counted as good deeds for its owner; and if it passes by a river and
drinks from it, then that will also be regarded as a good deed for its owner even if he has had no
intention of watering it then. Horses are a shelter from poverty to the second person who keeps horses
for earning his living so as not to ask others, and at the same time he gives Allah’s right (i.e. rak`at)
(from the wealth he earns through using them in trading etc.,) and does not overburden them. He who
keeps horses just out of pride and for showing off and as a means of harming the Muslims, his horses
will be a source of sins to him.” When Allah’s Messenger (ﷺ) was asked about donkeys, he replied, “Nothing
particular was revealed to me regarding them except the general unique verse which is applicable to
everything: “Whoever does goodness equal to the weight of an atom (or small ant) shall see it (its
reward) on the Day of Resurrection.”