Musnad Ahmad 26:609

Bahz narrated to me, Hammam narrated to us, Qatadah informed us, from Muwarriq, from Abu al-Ahwas al-Jushami, from Ibn Mas'ud, that the Prophet, peace and blessings of Allah be upon him, "used to prefer the prayer of the congregation over the prayer of a man alone by twenty-five prayers, all of them like his prayer."