Musnad Ahmad 30:549

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from Ibn al-Musayyib and Abu Salamah, or one of them, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "When one of you leads the people in prayer, let him lighten it, for among them are the weak, the elderly, and those with needs."