Musnad Abu Ya'la al-Mawsili 151:34

Hadbata narrated to us, Hammam bin Yahya narrated to us, Qatadah narrated to us, from Muwarriq al-Ijli, from Abu al-Ahwas, from Ibn Mas'ud, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The prayer of the congregation is superior to the prayer of a man alone by twenty-some parts."