Musnad Ahmad 30:466

Abu Kamil narrated to us, Ibrahim narrated to us, from Ibn Shihab. And Ya'qub narrated to us, my father narrated to us, from Ibn Shihab, from Sa'id ibn al-Musayyib, from Abu Hurayrah. Ibrahim said: I do not know it except from the Prophet, may Allah bless him and grant him peace. 'Abdullah ibn Ahmad said: My father said: "And Ya'qub did not doubt." He said: "The superiority of the congregational prayer over the prayer of one of you alone is twenty-five parts."