Sahih Muslim 1477
صحيح مسلم#1477SahihIbn Umar reported Allah's Messenger (ﷺ) assaying:Prayer said in a congregation is twenty-seven degrees more excellent than prayer said by a single person
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: 5:310