Musnad Ahmad 572:20

Mu'awiyah ibn 'Amr narrated to us, from Za'idah, from 'Abdullah ibn Khuthaym, he said: Dawud ibn Abi 'Asim al-Thaqafi narrated to me, from 'Uthman ibn Abi al-'As, that the last words the Messenger of Allah, peace and blessings of Allah be upon him, spoke to me when he appointed me over Ta'if were: "Lighten the prayer for the people, until I have set for you [the length of] 'Recite in the name of your Lord who created' (Surah al-'Alaq) and its likes from the Qur'an."