كتاب الصيام
13
The Book of Fasting
(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.
(2)
بَاب وُجُوبِ صَوْمِ رَمَضَانَ لِرُؤْيَةِ الْهِلَالِ وَالْفِطْرِ لِرُؤْيَةِ الْهِلَالِ وَأَنَّهُ إِذَا غُمَّ فِي أَوَّلِهِ أَوْ آخِرِهِ أُكْمِلَتْ عِدَّةُ الشَّهْرِ ثَلَاثِينَ يَوْمًا

Abu Huraira reported Allah's Messenger (ﷺ) as saying:

Observe fast on sighting it (the new moon) and break (fast) on sighting it (the new moon), but if the sky is cloudy for you, then complete the number (of thirty).
حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ سَلاَّمٍ الْجُمَحِيُّ، حَدَّثَنَا الرَّبِيعُ، - يَعْنِي ابْنَ مُسْلِمٍ - عَنْ مُحَمَّدٍ، - وَهُوَ ابْنُ زِيَادٍ - عَنْ أَبِي هُرَيْرَةَ، - رضى الله عنه - أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ ‏ "‏ صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ غُمِّيَ عَلَيْكُمْ فَأَكْمِلُوا الْعَدَدَ ‏"‏ ‏.‏
Reference : Sahih Muslim 1081 b
In-book reference : Book 13, Hadith 21
USC-MSA web (English) reference : Book 6, Hadith 2379
  (deprecated numbering scheme)