Sahih Muslim 2501
صحيح مسلم#2501SahihUbaidullah narrated on the authority of the same chain of transmitters that the Messenger of Allah (ﷺ) made a mention of Ramadan and said:The month may consist of twenty-nine days, and it may be thus, thus and thus, and (he further) said: Calculate it, but he did not say thirty
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: 13:7