Musnad Ahmad 299:8

Muhammad ibn Bakr narrated to us, Shu'bah narrated to us, from al-Nu'man ibn Salim, who said: I heard our elders from Thaqif, they said: Uthman ibn Abi al-'As informed us that he said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "Lead your people in prayer. And when you lead your people in prayer, make the prayer light for them, for among them are the young, the old, the weak, the sick, and those who have needs."