Musnad Ahmad 299:9

Waki’ narrated to us, ‘Amr ibn ‘Uthman narrated to us, from Musa ibn Talhah, from ‘Uthman ibn Abi al-‘As, who said: The Messenger of Allah (ﷺ) said to me: "O ‘Uthman, lead your people in prayer. Whoever leads the people, let him lighten [the prayer], for among them are the weak, the elderly, and those with needs. But if you pray by yourself, then pray however you wish."