Musnad Ahmad 1034:206

'Abd al-Samad narrated to us, Hammad narrated to us, from 'Asim, from Zirr, from Hudhayfah, that Gabriel met the Messenger of Allah, may Allah bless him and grant him peace, at the stones of al-Mira'. He said: "O Gabriel, I have been sent to an unlettered nation: to the old man and the old woman, the boy and the girl, and the old man who has never read a book at all." He said: "Indeed, the Qur'an was revealed in seven ahruf (modes/dialects)."