Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 303
Narrated Ibn Umar:
That he was riding a horse on the day, the Muslims fought (against the Byzantines), and the commander of the Muslim army was Khalid bin Al-Walid who had been appointed by Abu Bakr. The enemy took the horse away, and when the enemy was defeated, Khalid returned the horse to him.