Musnad Ahmad 25:1498

Waki’ narrated to us, Shu’bah narrated to us, from Abu Bishr, from Sa’id ibn Jubayr, from Ibn Abbas, who said: "The Prophet, peace and blessings of Allah be upon him, was taken (passed away) while I was ten years old, circumcised, and I had already recited the Muhkam (clear/precise) portions of the Quran."