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 2502 in Maktaba Shamila
وحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ ، حَدَّثَنَا إِسْمَاعِيلُ ، عَنْ أَيُّوبَ ، عَنْ نَافِعٍ ، عَنْ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا ، قَالَ : قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : " إِنَّمَا الشَّهْرُ تِسْعٌ وَعِشْرُونَ ، فَلَا تَصُومُوا حَتَّى تَرَوْهُ ، وَلَا تُفْطِرُوا حَتَّى تَرَوْهُ ، فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا لَهُ " .
Ibn'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 do not fast till you have sighted it (the new moon) and do not break fast, till you have sighted it (the new moon of Shawwal), and if the sky is cloudy for you, then calculate.
