Musnad Ahmad 27:153

Sufyan narrated to us, from Ibn Abi Najih, from Mujahid, who said: Ibn Umar witnessed the Conquest [of Mecca] while he was twenty years old, and with him was a stubborn horse and a heavy spear. Ibn Umar went to cut fodder for his horse. The Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, Abdullah; indeed, Abdullah."