Bukhari 4186
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Narrated Nafi`:The people used to say that Ibn `Umar had embraced Islam before `Umar. This is not true. What happened is that `Umar sent `Abdullah to bring his horse from an Ansari man so as to fight on it. At that time the people were giving the Pledge of allegiance to Allah's Messenger (ﷺ) near the Tree, and `Umar was not aware of that. So `Abdullah (bin `Umar) gave the Pledge of Allegiance (to the Prophet) and went to take the horse and brought it to `Umar. While `Umar was putting on the armor to get ready for fighting, `Abdullah informed him that the people were giving the Pledge of allegiance to Allah's Apostle beneath the Tree. So `Umar set out and `Abdullah accompanied him till he gave the Pledge of allegiance to Allah's Messenger (ﷺ), and it was this event that made people say that Ibn `Umar had embraced Islam before `Umar