Sahih Muslim 2505
صحيح مسلم#2505SahihIbn 'Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying:The month may consist of twenty-nine nights. So do not fast till you have sighted it (the new moon) and do not break it till you have sighted it, except when the sky is cloudy for you, and if it is so, 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: 13:11