Musannaf Ibn Abi Shayba 36:284

'Abd al-Rahim ibn Sulayman and Ibn Idris 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) reviewed me on the day of the Trench while I was fifteen years old, and he permitted me [to fight]. However, Ibn Idris said: "I was presented."