Musnad Ahmad 31:539

Ahmad ibn al-Hajjaj narrated to us, Abd al-Aziz ibn Abi Hazim informed us, Yazid ibn Abdullah ibn al-Had narrated to us, 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."