Musnad Abu Ya'la al-Mawsili 154:122

Muhammad ibn Abdullah al-Mukharrimi narrated to us, Muhammad ibn Sa’d al-Ash’ali al-Ansari narrated to us, he said: Ibn ‘Ajlan narrated to me, from Nu’aym ibn Abdullah al-Mujmir, from Ibn ‘Umar, from the Prophet (peace and blessings of Allah be upon him), who said: "The congregational prayer is superior to the prayer of the individual by twenty-seven parts."