People say that Ibn Umar accepted Islam before Umar, but that is not the case. Rather, on the day of Hudaybiyyah, Umar sent Abdullah to fetch a horse of his that was with a man of the Ansar, so that he could ride it into battle. Meanwhile, the Messenger of Allah ﷺ was receiving the pledge of allegiance under the tree, and Umar did not know of it. Abdullah gave his pledge, then went to fetch the horse and brought it to Umar, who was putting on his armor for battle. He informed him that the Messenger of Allah ﷺ was taking the pledge under the tree.He said, so Umar set out and went with him until he gave his pledge to the Messenger of Allah ﷺ. This is what people are referring to when they say that Ibn Umar accepted Islam before Umar.
إن الناس يتحدثون أن ابن عمر أسلم قبل عمر، وليس كذلك، ولكن عمر يوم الحديبية أرسل عبد الله إلى فرس له عند رجل من الأنصار، يأتي به ليقاتل عليه، ورسول الله ﷺ يبايع عندالشجرة، وعمر لا يدري بذلك، فبايعه عبد الله ثم ذهب إلى الفرس، فجاء به إلى عمر وعمر، يستلئم للقتال، فأخبره أن رسول الله ﷺ يبايع تحت الشجرة، قال: فانطلق فذهب معه حتى بايع رسول الله ﷺ، فهمي التي يتحدث الناس أن ابن عمر أسلم قبل عمر.
Bukhari narrated it in Al-Maghazi (4186), from Shuja bin Walid (he is the tutor, Abu Layth), who heard Nadr bin Muhammad, who reported to us from Sakhr, from Nafi, who said, then he mentioned it.The apparent form of this chain suggests it is disconnected, but in the narration that follows in Bukhari (4187), it appears that he took it from Ibn Umar.His saying, "he arms himself for battle," means he puts on the lamah, with a hamza, which is armor.