Sahih Bukhari Volume 4, Book 52, Hadith Number 303.

Narated By 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.