Musnad Ahmad 928:24

Husayn ibn Ali al-Ju'fi narrated to us, from Za'idah, from 'Asim, from Zirr, from Ubayy, who said: The Messenger of Allah (peace and blessings of Allah be upon him) met Jibril at the stones of al-Mira'. The Messenger of Allah (peace and blessings of Allah be upon him) said to Jibril: "I have been sent to an unlettered nation, among whom are the stubborn old man, the very old woman, and the young boy." He said: "So command them to recite the Quran according to seven ahruf (modes/dialects)."