The Messenger of Allah ﷺ said, "Goodness is tied to the forelocks of horses until the Day of Resurrection. Horses are of three kinds. For one man they are a reward, for another they are a covering, and upon a third they are a burden.As for the one for whom they are a reward, it is the man who keeps them ready for the cause of Allah and dedicates them to that purpose. Nothing disappears into their bellies without Allah recording for him a reward for everything that passes into them, even if he were to take them out to graze in a meadow."And he continued the hadith.
أخبرنا عمرو بن يحيى بن الحارث قال حدثنا محبوب بن موسى قال حدثنا أبو إسحق يعني الفزاري عن سهيل بن أبي صالح عن أبيه
قال رسول الله ﷺ الخيل معقود في نواصيها الخير إلى يوم القيامة الخيل ثلاثة فهي لرجل أجر وهي لرجل ستر وهي على رجل وزر فأما الذي هي له أجر فالذي يحتبسها في سبيل الله فيتخذها له ولا تغيب في بطونها شيئا إلا كتب له بكل شيء غيبت في بطونها أجر ولو عرضت له مرج وساق الحديث
Sourcing, its isnad is sahih. Tirmidhi transmitted it in the Book of the Merits of Jihad, chapter on whoever kept a horse tethered in the path of Allah, no. 1636, from the hadith of Suhayl, through him, and he said, hasan sahih. It is also in Kubra, no. 4402.