Musnad Abu Ya'la al-Mawsili 151:29

Abu Bakr ibn Abi Shaybah narrated to us, Muhammad 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 superiority of a man’s prayer in congregation over his prayer alone is twenty-some degrees."