Sahih Muslim #2500 · The Book of Fasting

Sahih MuslimThis hadith is narrated on the authority of 'Ubaidullah with the same chain of transmitters
Sahih Muslim · Book 13 · The Book of Fasting
Sahih Muslim · كتاب 13 · كتاب الصيام
If (the sky) is cloudy for you, then calculate thirty days (for the month of Ramadan).
وَحَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عُبَيْدُ اللَّهِ، بِهَذَا الإِسْنَادِ وَقَالَ ‏
"‏ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا ثَلاَثِينَ ‏"‏ ‏.‏ نَحْوَ حَدِيثِ أَبِي أُسَامَةَ ‏.‏
Reference : Sahih Muslim 2500In-book reference : Book 13, Hadith 6