Musnad Ahmad 30:2991

Yahya ibn Sa'id narrated to us, from Malik, who said: Al-Zuhri narrated to me, from Sa'id ibn al-Musayyib, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "The excellence of the congregational prayer over a man's prayer alone is twenty-five parts." Yahya said: If Allah wills.