Musnad Ahmad 26:608

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, and Hajjaj narrated to us, he said: Shu‘bah narrated to me, Hajjaj said: I heard ‘Uqbah ibn Wasaj, from Abu al-Ahwas, from ‘Abdullah, from the Prophet, may Allah bless him and grant him peace, that he said: "The excellence of a man’s prayer in congregation over his prayer alone is twenty-five degrees." Hajjaj said: "Shu‘bah did not attribute it to the Prophet (marfu‘) to me, although he did attribute it to others besides me. He said: 'I am hesitant to attribute it to the Prophet, because ‘Abdullah rarely used to attribute [statements] to the Prophet, may Allah bless him and grant him peace.'"