Bukhari

Hadith Commentary:
1.
At the end of the narration, the statement of Abu Hurairah radi Allahu anhu is also reported as a marfu‘ (attributed to the Prophet) hadith in another narration, in which the Messenger of Allah sallallahu alayhi wa sallam said:
“A horse is a source of reward for the person who ties it in the path of Allah and lets its rope loose in a pasture or garden; whatever it eats from that pasture or garden will be counted as good deeds for him.
If it breaks its rope and runs over one or two elevations or hills, then its dung and the marks of its hooves will also be counted as good deeds for that person.
If it passes by a river and drinks from it, even though its owner did not intend to give it water, this too will be counted as good deeds for him.
Such a horse is a source of reward.”
(Sahih al-Bukhari, al-Jihad, Hadith 2860)

2.
The purpose is that when a mujahid sets out for jihad, then whatever he does, day or night, whether sleeping or awake, he will receive reward for it—whether he does it himself, or his servant or laborer does it, or any of his animals does it.
This virtue is specific to the act of jihad and does not apply to other acts of obedience, because a person performing prayer or fasting receives reward only as long as he is engaged in prayer or fasting, whereas for the mujahid, the chain of reward continues for twenty-four hours.