Musnad Ahmad 30:2732

Hajjaj narrated to us, he said: Shareek narrated to us, from Ash'ath ibn Sulaym, from Abu al-Ahwas, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, that he said: "The prayer in congregation is superior to the prayer alone by twenty-seven degrees, or twenty-five degrees."