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 2503 in Maktaba Shamila
وحَدَّثَنِي حُمَيْدُ بْنُ مَسْعَدَةَ الْبَاهِلِيُّ ، حَدَّثَنَا بِشْرُ بْنُ الْمُفَضَّلِ ، حَدَّثَنَا سَلَمَةُ وَهُوَ ابْنُ عَلْقَمَةَ ، عَنْ نَافِعٍ ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا ، قَالَ : قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : " الشَّهْرُ تِسْعٌ وَعِشْرُونَ ، فَإِذَا رَأَيْتُمُ الْهِلَالَ فَصُومُوا ، وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا ، فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا لَهُ " .
'Abdullah bin 'Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying: The month (of Ramadan) may consist of twenty nine days; so when you see the new moon observe fast and when you see (the new moon again at the commencement of the month of Shawwal) then break It, and if the sky is cloudy for you, then calculate it (and complete thirty days).
