Musannaf Ibn Abi Shayba 3:2282

Waki' narrated to us, from 'Amr ibn 'Uthman ibn Mawhab, from Musa ibn Talha, from 'Uthman ibn Abi al-'As, that the Prophet, may Allah bless him and grant him peace, said to him: "Lead your people in prayer. Whoever leads a people in prayer, let him lighten it, for among them are the weak, the elderly, and those with needs. But when you pray by yourself, pray however you wish."