Sahih Muslim | Book 20 | Chapter 23

Narrated by It has been narrated on the authority of Ibn 'Umar who
It has been narrated on the authority of Ibn 'Umar who said: The Messenger of Allah (may peace be upon him) 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 years old, 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.

Narrated by
This tradition has been handed down through a different chain Of transmitters with the following change in the wording: "I was fourteen years old and he thought me too young (to participate in the fight).