Musnad Abu Ya'la al-Mawsili 1:106

Al-Abbas ibn al-Walid al-Narsi narrated to us, Abu al-Ahwas narrated to us, from Abu Ishaq, from Ikrimah, who said: Abu Bakr said: I asked the Messenger of Allah, may Allah bless him and grant him peace, "What has caused you to turn grey?" And he mentioned the like of it.