Musnad Abu Ya'la al-Mawsili 121:1

Zuhayr and Harun ibn Ma'ruf narrated to us, they said: Ibn al-Darawardi narrated to us, from Rabi'ah, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) was sent [as a prophet] at the age of forty, and he passed away while he was sixty years old. There were not twenty white hairs in his beard or on his head.