Sahih Muslim 2499
صحيح مسلم#2499Sahihحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا أَبُو أُسَامَةَ، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، رضى الله عنهما أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم ذَكَرَ رَمَضَانَ فَضَرَبَ بِيَدَيْهِ فَقَالَ " الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا - ثُمَّ عَقَدَ إِبْهَامَهُ فِي الثَّالِثَةِ - فَصُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ أُغْمِيَ عَلَيْكُمْ فَاقْدِرُوا لَهُ ثَلاَثِينَ " .
Ibn Umar reported that Allah's Messenger (ﷺ) made a mention of Ramadan and he with the gesture of his hand said:The month is thus and thus. (He then withdrew his thumb at the third time). He then said: Fast when you see it, and break your fast when you see it, and if the weather is cloudy calculate it (the months of Sha'ban and Shawwal) as thirty days
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 13:5