وَعَنِ اِبْنِ عُمَرَ رَضِيَ اَللَّهُ عَنْهُمَا
قَالَ: «عُرِضْتُ عَلَى اَلنَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَوْمَ أُحُدٍ، وَأَنَا اِبْنُ أَرْبَعَ عَشْرَةَ سَنَةً، فَلَمْ يُجِزْنِي، وَعُرِضْتُ عَلَيْهِ يَوْمَ اَلْخَنْدَقِ، وَأَنَا اِبْنُ خَمْسَ عَشْرَةَ سَنَةً، فَأَجَازَنِي»
I was brought before the Prophet (sallallahu alayhi wa sallam) on the Day of Uhud when I Was fourteen years old, but he did not give me permission (to fight). I was afterwards brought to him on the Day of al-Khandaq (the Battle of Trench) when I was fifteen years of age, and he gave me permission (to fight).
Bulugh al-Maram 998, Book 7, Business Transactions, Hadith 109. 1 min read