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 2504 in Maktaba Shamila
حَدَّثَنِي حَرْمَلَةُ بْنُ يَحْيَى ، أَخْبَرَنَا ابْنُ وَهْبٍ ، أَخْبَرَنِي يُونُسُ ، عَنْ ابْنِ شِهَابٍ ، قَالَ : حَدَّثَنِي سَالِمُ بْنُ عَبْدِ اللَّهِ ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا ، قَالَ : سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ : " إِذَا رَأَيْتُمُوهُ فَصُومُوا ، وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا ، فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا لَهُ " .
''Abdullah bin 'Umar (RA) (Allah be pleased with both of them) reported Allah's Measenger (ﷺ) as saying: When you see the new moon, observe fast, and when you see it (again) then break it, and if the sky is cloudy for you, then calculate it.
