From Sahih Bukhari (Hadith 3068):
Narrated 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
Reference: Bukhari 3068