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 2499 in Maktaba Shamila
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا أَبُو أُسَامَةَ ، حَدَّثَنَا عُبَيْدُ اللَّهِ ، عَنْ نَافِعٍ ، عَنْ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا ، أَنّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ذَكَرَ رَمَضَانَ ، فَضَرَبَ بِيَدَيْهِ ، فَقَالَ : " الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا ، ثُمَّ عَقَدَ إِبْهَامَهُ فِي الثَّالِثَةِ ، فَصُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ ، فَإِنْ أُغْمِيَ عَلَيْكُمْ فَاقْدِرُوا لَهُ ثَلَاثِينَ " ،
Ibn 'Umar (RA) reported that Allah's Messenger (ﷺ) made a mention of Ramadan and he with the gesture of his hand said: The month is thus and thus. (He then withdrew his thumb at the third time). He then said: Fast when you see it, and break your fast when you see it, and if the weather is cloudy calculate it (the months of Sha'ban and Shawwal) as thirty days.
