Musnad Ahmad 30:3169

'Abd al-Malik ibn 'Amr narrated to us, he said: Aflah ibn Humayd narrated to us, from Abu Bakr ibn Hazm, from Salman al-Agharr, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, who said: "A prayer in this mosque of mine is like a thousand prayers in what is other than it of the mosques, except for the Sacred Mosque (al-Masjid al-Haram). And the prayer of the congregation is equivalent to twenty-five of the prayer of the individual."