Musnad Ahmad 30:2028

Ibrahim ibn Abi al-Abbas narrated to us, he said: Abu Uways narrated to us, from al-Zuhri, from Sa'id ibn al-Musayyib, from Abu Hurayrah, from the Messenger of Allah, peace and blessings of Allah be upon him, who said: "The superiority of the congregational prayer over the prayer of one of you alone is twenty-five parts."