Sahih Muslim 2498
صحيح مسلم#2498SahihIbn Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying in connection with Ramadan:Do not fast till you see the new moon, and do not break fast till you see it; but if the weather is cloudy calculate about it
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: 13:4