The Book of Fasting
13/2. 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.
Sahih Muslim
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا مُحَمَّدُ بْنُ بِشْرٍ الْعَبْدِيُّ، حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ، عُمَرَ عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، - رضى الله عنه - قَالَ ذَكَرَ رَسُولُ اللَّهِ صلى الله عليه وسلم الْهِلاَلَ فَقَالَ " إِذَا رَأَيْتُمُوهُ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ أُغْمِيَ عَلَيْكُمْ فَعُدُّوا ثَلاَثِينَ " .
Abu Huraira (Allah be pleased with him) narrated that the Messenger of Allah (may peace he upon him) made a mention of the new moon and (in this connection) said: Observe fast when you see it (the new moon) and break fast when you see it (the new moon of Shawwal), but when (the actual position of the month is) concealed from you (on account of cloudy sky), then count thirty days.
Reference : Sahih Muslim 1081dIn-book reference : Book 13, Hadith 23USC-MSA web (English) reference : Book 6, Hadith 2381