Musnad Abu Ya'la al-Mawsili 152:8

Muhammad ibn Abdullah ibn Numayr narrated to us, Ibn Fudayl narrated to us, from Ata’ ibn al-Sa’ib, from Abu al-Ahwas, from Abdullah, that he heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "The prayer of a man in congregation exceeds his prayer alone by twenty-some degrees."