Musnad Ahmad 25:781

'Affan narrated to us, Yazid bin Zuray' narrated to us, Yunus narrated to us, from 'Ammar, the freed slave of Banu Hashim, who said: I asked Ibn 'Abbas, "How old was the Messenger of Allah, may Allah bless him and grant him peace, on the day he died?" He said, "I did not think that someone like you, among his people, would be unaware of that." I said, "I have asked others, and they differed with me, so I wanted to know your opinion on it." He said, "Do you calculate?" I said, "Yes." He said, "Hold: He was forty when he was sent [as a Prophet], he stayed in Makkah for fifteen years, secure and fearful, and he was in Madinah for ten years as an emigrant."