Sahih Muslim 1478
صحيح مسلم#1478SahihIbn 'Umar reported Allah's Apostle (ﷺ) as saying:The prayer of a person in congregation is twenty-seven times in excess to the prayer said 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 Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 5:311