If somebody keeps a horse in Allah's cause motivated by his faith in Allah and his belief in His promise, then he will be rewarded on the Day of Resurrection for what the horse has eaten or drunk and for its dung and urine

Explanation of the Hadīth: Allah promised in the Qur'an: {And whatever you spend of good - it will be fully repaid to you} [Sūrat al-Baqarah: 272]. So, anyone dedicates a horse to fighting in the cause of Allah, seeking His reward and pleasure and believing in His promise, Allah will definitely reward such a person for everything that the horse eats, drinks, or excretes from urine and dung. Allah will place all that in the scale of his good deeds on the Day of Judgment.

In a lengthy Hadīth reported by Abu Hurayrah (may Allah be pleased with him) the Prophet (may Allah's peace and blessings be upon him) said: "Horses are kept for one of three purposes; for some people they are a source of sin, for some others they are a means of shelter, and for some others they are a source of reward. They are a source of reward for the person who keeps them for the cause of Allah tying them with a long tether on a meadow or in a garden. The result is that whatever the horses eat from the area of the meadow or the garden where they are tied and whatever they excrete from dung and urine will be counted as good deeds for his benefit. If they break the rope and jump over one or two hillocks, then all their footprints and dung will be written as good deeds for him. If they pass by a river with their owner and drink water from it, even though he had no intention of watering them, then he will get reward as much as the water drunk.”