Sahih Muslim 2506
صحيح مسلم#2506SahihIbn Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying:The month is thus and thus and thus (i. e. pointing with his fingers thrice), and he held back his thumb at the third time (in order to show that it can also 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: 13:12