مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4131
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdullah that The Prophet (ﷺ) said: “A man's prayer in congregation is twenty-five times better than his praying alone.” Hajjaj said: Shu'bah did not attribute it to the Prophet (ﷺ) (when he narrated it) to me, but he did so (when he narrated it) to others. And I am reluctant to attribute it to the Prophet (ﷺ) because 'Abdullah rarely attributed any report to the Prophet (ﷺ).
Musnad Ahmad 4158A saheeh hadeeth]