Musnad Abu Ya'la al-Mawsili 42:38

Abu Bakr narrated to us, Abu Mu'awiyah narrated to us, from Hilal ibn Maymun, from 'Ata' ibn Yazid, from Abu Sa'id, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "A man's prayer in congregation exceeds his prayer alone by twenty-five degrees. And if he prays it in a land, and perfects its ablution, its bowing, and its prostration, his prayer reaches fifty degrees."