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
Sahih Muslim 1080 k

Abdullah b. 'Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying:

The month (of Ramadan) is thus and thus, and thus. i.e. ten, ten and nine.
وَحَدَّثَنَا سَهْلُ بْنُ عُثْمَانَ، حَدَّثَنَا زِيَادُ بْنُ عَبْدِ اللَّهِ الْبَكَّائِيُّ، عَنْ عَبْدِ الْمَلِكِ بْنِ عُمَيْرٍ، عَنْ مُوسَى بْنِ طَلْحَةَ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، - رضى الله عنهما - عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا عَشْرًا وَعَشْرًا وَتِسْعًا ‏"‏ ‏.‏
Reference : Sahih Muslim 1080k
In-book reference : Book 13, Hadith 14
USC-MSA web (English) reference : Book 6, Hadith 2373
  (deprecated numbering scheme)