Sahih Bukhari Volume 4, Book 52, Hadith Number 302.

Narated By Nafi : Once a slave of Ibn ‘Umar fled and joined the Byzantine. Khalid bin Al-Walid got him back and returned him to ‘Abdullah (bin ‘Umar). Once a horse of Ibn ‘Umar also ran away and followed the Byzantines, and he (i.e. Khalid) got it back and returned it to ‘Abdullah.