Musannaf Ibn Abi Shayba 3:6013

Abu Mu'awiyah narrated to us, from Hisham bin Maymun, from 'Ata' bin Yazid al-Laythi, from Abu Sa'id al-Khudri, 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 desolate land, and perfects its ablution, its bowing, and its prostration, his prayer reaches fifty degrees."