Musannaf Ibn Abi Shayba 28:1170

'Abd al-Rahim ibn Sulayman narrated to us, from 'Ubayd Allah ibn 'Umar, from Nafi', from Ibn 'Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) presented me for the fighting while I was fourteen years old, and he deemed me too young, so he turned me away. Then he presented me on the day of the Trench while I was fifteen years old, and he permitted me. Nafi' said: I narrated that to 'Umar ibn 'Abd al-'Aziz while he was the Caliph, and he said: "Indeed, this is the boundary between the young and the adult." So he wrote to his governors that whoever reached fifteen, enroll him among the combatants, and whoever was younger than that, enroll him among the [non-combatant] fighters.