Sahih Muslim 1868 a
It has been narrated on the authority of Ibn 'Umar who said:
The Messenger of Allah (ﷺ) inspected me on the battlefield on the Day of Uhud, and I was fourteen years old. He did not allow me (to take part in the fight). He inspected me on the Day of Khandaq-and I was fifteen yearsold, and he permitted me (to fight), Nafi' said: I came to 'Umar b. 'Abd al-'Aziz who was then Caliph, and narrated this tradition to him. He said: Surely, this is the demarcation between a minor and a major. So he wrote to his governors that they should pay subsistence allowance to one who was fifteen years old, but should treat those of lesser age among children.
حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ عَرَضَنِي رَسُولُ اللَّهِ صلى الله عليه وسلم يَوْمَ أُحُدٍ فِي الْقِتَالِ وَأَنَا ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَلَمْ يُجِزْنِي وَعَرَضَنِي يَوْمَ الْخَنْدَقِ وَأَنَا ابْنُ خَمْسَ عَشْرَةَ سَنَةً فَأَجَازَنِي ‏.‏ قَالَ نَافِعٌ فَقَدِمْتُ عَلَى عُمَرَ بْنِ عَبْدِ الْعَزِيزِ وَهُوَ يَوْمَئِذٍ خَلِيفَةٌ فَحَدَّثْتُهُ هَذَا الْحَدِيثَ فَقَالَ إِنَّ هَذَا لَحَدٌّ بَيْنَ الصَّغِيرِ وَالْكَبِيرِ ‏.‏ فَكَتَبَ إِلَى عُمَّالِهِ أَنْ يَفْرِضُوا لِمَنْ كَانَ ابْنَ خَمْسَ عَشْرَةَ سَنَةً وَمَنْ كَانَ دُونَ ذَلِكَ فَاجْعَلُوهُ فِي الْعِيَالِ ‏.‏