Sahih Muslim 2517
صحيح مسلم#2517SahihAbu Huraira (Allah be pleased with him) narrated that the Messenger of Allah (may peace he upon him) made a mention of the new moon and (in this connection) said:Observe fast when you see it (the new moon) and break fast when you see it (the new moon of Shawwal), but when (the actual position of the month is) concealed from you (on account of cloudy sky), then count thirty 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: 13:23