Musnad Ahmad 31:531

Harun ibn Ma'ruf narrated to us, Ibn Wahb narrated to us, Haywah said: Ibn al-Had narrated to me, from Abdullah ibn Khabbab, from Abu Sa'id al-Khudri, that he heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The congregational prayer is superior to the prayer of the individual by twenty-five degrees."