Musnad Ahmad 299:10

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Amr ibn Murrah, he said: I heard Sa‘id ibn al-Musayyib, he said: ‘Uthman ibn Abi al-‘As narrated, he said: The last thing the Messenger of Allah, peace and blessings of Allah be upon him, enjoined upon me was: "When you lead a people in prayer, make the prayer light for them."