Sahih
Muslim
صحيح مسلم
13
The Book of Fasting
كتاب الصيام
Chapter 2: 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.
بَاب وُجُوبِ صَوْمِ رَمَضَانَ لِرُؤْيَةِ الْهِلَالِ وَالْفِطْرِ لِرُؤْيَةِ الْهِلَالِ وَأَنَّهُ إِذَا غُمَّ فِي أَوَّلِهِ أَوْ آخِرِهِ أُكْمِلَتْ عِدَّةُ الشَّهْرِ ثَلَاثِينَ يَوْمًا
Sahih Muslim 1080 g
'Abdullah b. Umar (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.حَدَّثَنِي حَرْمَلَةُ بْنُ يَحْيَى، أَخْبَرَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، قَالَ حَدَّثَنِي سَالِمُ بْنُ عَبْدِ اللَّهِ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، - رضى الله عنهما - قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " إِذَا رَأَيْتُمُوهُ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا لَهُ " .
Grade: Sahih