Musnad Ahmad 30:1983

Husayn narrated to us, Ibn Abi Dhi’b narrated to us, from Abu al-Walid, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "When you lead the people in prayer, then make it light, for among them are the elderly, the weak, and the young." And he said in another narration: From Abu al-Walid, the freed slave of ‘Amr ibn Khidash.