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 أَبُو أُسَامَةَ، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، رضى الله عنهما أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم ذَكَرَ رَمَضَانَ فَضَرَبَ بِيَدَيْهِ فَقَالَ " الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا - ثُمَّ عَقَدَ إِبْهَامَهُ فِي الثَّالِثَةِ - فَصُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ أُغْمِيَ عَلَيْكُمْ فَاقْدِرُوا لَهُ ثَلاَثِينَ " .
Ibn Umar 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.
Reference : Sahih Muslim 1080bIn-book reference : Book 13, Hadith 5USC-MSA web (English) reference : Book 6, Hadith 2364