Musnad Ahmad 26:20

Muhammad ibn Abi ‘Adi narrated to us, from Sa‘id, from Qatadah, from Abu al-Ahwas, from ‘Abdullah ibn Mas‘ud, that the Prophet of Allah, peace and blessings of Allah be upon him, said: "The prayer of the congregation is superior to the prayer of a man alone by twenty-five degrees; all of it is like his prayer."