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

Ibn Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying in connection with Ramadan:

Do not fast till you see the new moon, and do not break fast till you see it; but if the weather is cloudy calculate about it.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، - رضى الله عنهما - عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ ذَكَرَ رَمَضَانَ فَقَالَ ‏ "‏ لاَ تَصُومُوا حَتَّى تَرَوُا الْهِلاَلَ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ أُغْمِيَ عَلَيْكُمْ فَاقْدِرُوا لَهُ ‏"‏ ‏.‏
Reference : Sahih Muslim 1080 a
In-book reference : Book 13, Hadith 4
USC-MSA web (English) reference : Book 6, Hadith 2363
  (deprecated numbering scheme)