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 Apostle 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 Apostle, and it was this event that made people say that Ibn 'Umar had embraced Islam before 'Umar. "Abdullah bin 'Umar added, "The people were along with the Prophet on the day of Al-Hudaibiya spreading in the shade of the trees. Suddenly the people surrounded the Prophet and started looking at him." 'Umar said, "O 'Abdullah! Go and see why the people are encircling Allah's Apostle and looking at him." 'Abdullah bin Umar then saw the people giving the Pledge o allegiance to the Prophet. So he also gave the Pledge of allegiance and returned to 'Umar who went out in his turn and gave the Pledge of allegiance to the Prophet.'
حَدَّثَنِي شُجَاعُ بْنُ الْوَلِيدِ، سَمِعَ النَّضْرَ بْنَ مُحَمَّدٍ، حَدَّثَنَا صَخْرٌ، عَنْ نَافِعٍ، قَالَ إِنَّ النَّاسَ يَتَحَدَّثُونَ أَنَّ ابْنَ عُمَرَ، أَسْلَمَ قَبْلَ عُمَرَ، وَلَيْسَ كَذَلِكَ، وَلَكِنْ عُمَرُ يَوْمَ الْحُدَيْبِيَةِ أَرْسَلَ عَبْدَ اللَّهِ إِلَى فَرَسٍ لَهُ عِنْدَ رَجُلٍ مِنَ الأَنْصَارِ يَأْتِي بِهِ لِيُقَاتِلَ عَلَيْهِ، وَرَسُولُ اللَّهِ صلى الله عليه وسلم يُبَايِعُ عِنْدَ الشَّجَرَةِ، وَعُمَرُ لاَ يَدْرِي بِذَلِكَ، فَبَايَعَهُ عَبْدُ اللَّهِ، ثُمَّ ذَهَبَ إِلَى الْفَرَسِ، فَجَاءَ بِهِ إِلَى عُمَرَ، وَعُمَرُ يَسْتَلْئِمُ لِلْقِتَالِ، فَأَخْبَرَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم يُبَايِعُ تَحْتَ الشَّجَرَةِ ـ قَالَ ـ فَانْطَلَقَ فَذَهَبَ مَعَهُ حَتَّى بَايَعَ رَسُولَ اللَّهِ صلى الله عليه وسلم، فَهِيَ الَّتِي يَتَحَدَّثُ النَّاسُ أَنَّ ابْنَ عُمَرَ أَسْلَمَ قَبْلَ عُمَرَ.
• Sahih al-Bukhari, 4187
• Sahih al-Bukhari, Vol. 5, Book of Military Expeditions led by the Prophet ﷺ (Maghaazi), Hadith 500
• Sahih al-Bukhari, Book of Military Expeditions led by the Prophet ﷺ (Maghaazi), Hadith 500