Nafi reported,

A slave belonging to Ibn Umar ran away and joined the Romans. Khalid bin Walid gained the upper hand over them and returned the slave to Abdullah. A horse of Ibn Umar's also bolted and joined the Romans, and when they were defeated, the horse was returned to Abdullah.Abu Abdullah said, "Aara is derived from ayr, which is a wild donkey, meaning it fled."

Bukhari 3068

Sahih

حدثنا محمد بن بشار حدثنا يحيى عن عبيد الله قال

أخبرني نافع

أن عبدا لابن عمر أبق فلحق بالروم فظهر عليه خالد بن الوليد فرده على عبد الله وأن فرسا لابن عمر عار فلحق بالروم فظهر عليه فردوه على عبد الله قال أبو عبد الله عار مشتق من العير وهو حمار وحش أي هرب

البخاري 3068صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Bukhari
Book
Jihad and expeditions
Chapter
If the polytheists seize the property of a Muslim then the Muslim finds it
Numbering
Darussalam 3068
Bukhari 3068