Sahih al-Bukhari 649
صحيح البخاري#649SahihNarrated `Abdullah bin `Umar:The reward of the congregational prayer is twenty seven times greater (than that of the prayer offered by a person alone)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 10:46