Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever keeps a horse in the path of Allah, believing in Allah and affirming His promise, then for its eating its fill and drinking its fill, its urine and excrement, that is, its dung, for all of these he will receive good deeds on the Day of Resurrection." In the narration of Ibn Wahb it is: "believing in Allah and affirming the promise of Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السبق والرمي / 19746