Musnad Ahmad 572:18

'Affan narrated to us, he said: Wuhayb narrated to us, he said: 'Abdullah ibn 'Uthman narrated to us, from Dawud ibn Abi 'Asim, from 'Uthman ibn Abi al-'As, that the last thing the Messenger of Allah, peace and blessings of Allah be upon him, entrusted to him was that he said: "If you lead a people in prayer, then make it light for them, until he specified for me: 'Recite: In the name of your Lord who created.'"