Musnad Abu Ya'la al-Mawsili 152:122

Abu Khaythama 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: "Indeed, the superiority of a man’s prayer in congregation exceeds his prayer alone by twenty-some degrees."