Jami` at-Tirmidhi 1361

Narrated Nafi':that Ibn 'Umar said: "I was reviewed before the Messenger of Allah (ﷺ) in the army, and I was fourteen years old, but he did not accept me. Then I was reviewed before him in the army later while I was fifteen years old, and he accepted me." Nafi' said: "I narrated this Hadith to 'Uman bin 'Abdul Azeez and he said: 'This is the limit that distinguishes between childhood and adulthood.' Then he wrote to give salaries to whoever reached fifteen years old." (Another chain) from Nafi', from Ibn 'Umar from the Prophet (ﷺ) and it is similar, but he did not mention in it that 'Umar bin 'Abdul-'Aziz wrote that this is the limit that distinguishes between youth and childhood and adulthood. In his narration, Ibn 'Uyainah said (that Nafi' said): "I narrated it to 'Umar bin 'Abdul-'Aziz and he said: 'This is the limit that distinguishes between children and soldiers