Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 105
Narrated Abu Huraira:
The Prophet said, "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."