Sahih Muslim 2353a

Ammar, the freed slave of Banu Hashim, reported:I asked Ibn 'Abbas how old was he when death overtook the Messenger of Allah (ﷺ). He said: I little know that such a thing is not known to a man like you who belong to his people. He said: I asked people about it but they differed with me, and I liked to know your opinion about it. He said: Do you know counting? He said: Yes. He then said: Bear this in mind very well that he was commissioned (as a Prophet) at the age of forty, and he stayed in Mecca for fifteen years; sometime in peace and sometime in dread, and (lived) for ten years after his migration to Medina