Musnad Ahmad 572:3

Yahya ibn Sa'id narrated to us, he said: 'Amr ibn 'Uthman narrated to us, Musa ibn Talhah narrated to me, that 'Uthman ibn Abi al-'As narrated to him that the Prophet, peace and blessings of Allah be upon him, commanded him to lead his people in prayer. He said: Then he said: "Whoever leads a people in prayer, let him lighten it, for among them are the weak, the elderly, the sick, and those with needs. But if he prays alone, let him pray however he wishes."