Musannaf Ibn Abi Shayba 3:6022

Abu Khalid al-Ahmar narrated to us, from ‘Amr ibn Qays, from ‘Ikrimah, from Ibn ‘Abbas, who said: The superiority of the congregational prayer over the prayer of the individual is twenty-five degrees. If they are more, then it is according to the number of those who are in the mosque. A man said: "Even if they are ten thousand?" He said: "Yes, even if they are forty thousand."