Musnad Abu Ya'la al-Mawsili 155:316

Amr al-Naqid narrated to us, Waki' narrated to us, from Aflah ibn Humayd, from Abu Bakr ibn Amr ibn Hazm, from Salman al-Agharr, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The congregational prayer exceeds the prayer of the individual by twenty-five degrees."