Sahih Bukhari
One-fifth of Booty to the Cause of Allah (Khumus)
Volume: 004 / Book Number: 053 / Hadith Number: 348
Narrated 'Urwa-al-Bariqi:
The Prophet said, "Horses are always the source of good, namely, rewards (in the Hereafter) and booty, till the Day of Resurrection."