Sahih Muslim 2500
صحيح مسلم#2500Sahihوَحَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عُبَيْدُ اللَّهِ، بِهَذَا الإِسْنَادِ وَقَالَ " فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا ثَلاَثِينَ " . نَحْوَ حَدِيثِ أَبِي أُسَامَةَ .
This hadith is narrated on the authority of 'Ubaidullah with the same chain of transmitters, and he said:If (the sky) is cloudy for you, then calculate thirty days (for the month of Ramadan)
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: 13:6