Sahih Muslim Book 20, Hadith Number 4605.

Chapter : The age of majority.

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.