صحيح مسلم
Sahih Muslim
كتاب الصيام
The Book of Fasting
باب وُجُوبِ صَوْمِ رَمَضَانَ لِرُؤْيَةِ الْهِلاَلِ وَالْفِطْرِ لِرُؤْيَةِ الْهِلاَلِ وَأَنَّهُ إِذَا غُمَّ فِي أَوَّلِهِ أَوْ آخِرِهِ أُكْمِلَتْ عِدَّةُ الشَّهْرِ ثَلاَثِينَ يَوْمًا:
Chapter: The obligation to fast Ramadan when the crescent is sighted, and to break the fast when the crescent is sighted, and that if it is cloudy at the beginning or end of the month, then the month should be completed as thirty days.
Hadith 1080
This hadith is listed as number 2500 in Maktaba Shamila
وحَدَّثَنَا ابْنُ نُمَيْرٍ ، حَدَّثَنَا أَبِي ، حَدَّثَنَا عُبَيْدُ اللَّهِ بِهَذَا الْإِسْنَادِ ، وَقَالَ : " فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا ثَلَاثِينَ " ، 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).
