Sahih Muslim 1051
صحيح مسلم#1051SahihUthman b. Abu'l-'As reported:The last thing which the Messenger of Allah (ﷺ) instructed me was: When you lead the people in prayer, be brief
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: 4:213