Sahih Muslim 2509
صحيح مسلم#2509SahihIbn Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying:The month is thus, and thus, and thus, and he flapped his hands with all their fingers twice. but at the third turn, folded his right thumb or left thumb (in order to give an idea of twenty-nine)
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: 13:15