Sahih Muslim 2504
صحيح مسلم#2504SahihAbdullah b. Umar (Allah be pleased with both of them) reported Allah's Measenger (ﷺ) as saying:When you see the new moon, observe fast, and when you see it (again) then break it, and if the sky is cloudy for you, then calculate 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: 13:10