Musannaf Ibn Abi Shayba 29:148

'Abdullah ibn Idris narrated to us, from 'Ubaydullah, from Nafi', from Ibn 'Umar, who said: I was presented to the Messenger of Allah (may Allah bless him and grant him peace) on the day of Uhud, and I was fourteen years old, so he considered me too young. And I was presented to him on the day of the Trench (al-Khandaq), and I was fifteen years old, so he permitted me.