Sunan Ibn Majah, Volume 3, Book 20, Hadith 2543 (DarusSalam) ⓘ

It was narrated that Ibn`Umar said: “I was presented to the Messenger of Allah on the day of Uhud, when I was fourteen years old, but he did not permit me (to fight). I was presented to him on the Day of Khandaq when I was fifteen years old, and he permitted me (to fight).' ”

حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ نُمَيْرٍ، وَأَبُو مُعَاوِيَةَ وَأَبُو أُسَامَةَ قَالُوا حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عُمَرَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ عُرِضْتُ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم يَوْمَ أُحُدٍ وَأَنَا ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَلَمْ يُجِزْنِي وَعُرِضْتُ عَلَيْهِ يَوْمَ الْخَنْدَقِ وَأَنَا ابْنُ خَمْسَ عَشْرَةَ سَنَةً فَأَجَازَنِي ‏.‏ قَالَ نَافِعٌ فَحَدَّثْتُ بِهِ عُمَرَ بْنَ عَبْدِ الْعَزِيزِ فِي خِلاَفَتِهِ فَقَالَ هَذَا فَصْلُ مَا بَيْنَ الصَّغِيرِ وَالْكَبِيرِ ‏.‏