Musnad Ahmad 30:3025

Waki' narrated to us, he said: Aflah ibn Humayd narrated to us, from Abu Bakr ibn Muhammad ibn 'Amr ibn Hazm, from al-Agharr, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "A man's prayer in congregation exceeds the prayer of the individual by twenty-five degrees."