Musnad Ahmad 27:214

Yahya narrated to us, from Ubaydullah, Nafi’ informed me, from Ibn Umar: "That the Prophet, peace and blessings of Allah be upon him, reviewed him on the day of Uhud, and he was fourteen years old, so he did not permit him. Then he reviewed him on the day of the Trench (al-Khandaq), and he was fifteen years old, so he permitted him."