Sahih Muslim 2507
صحيح مسلم#2507SahihIbn Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying:The month may consist of twenty-nine days
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: 13:13