Musnad Imam Shafi'i

Ibn Umar (may Allah be pleased with them both) narrated that I was presented before the Prophet (ﷺ) in the year of the Battle of Uhud and I was 14 years old, but he (ﷺ) did not accept me (for participation in the battle). Then I was presented before him in the year of the Battle of Khandaq when I was 15 years old, and he (ﷺ) gave me permission. Nafi‘ (may Allah have mercy on him) said: I narrated this hadith to ‘Umar bin ‘Abdul Aziz (may Allah have mercy on him), and he said: “This is the distinction (limit) between those who fight and the boys.” And he wrote that those who are 15 years old should be given a share of the spoils of war like the fighters, and those who have not reached 15 years should be counted among the boys.