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 l

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

The month is thus, and thus, and thus, and he flapped his hands with all their fingers twice. but at the third turn, folded his right thumb or left thumb (in order to give an idea of twenty-nine).
وَحَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا شُعْبَةُ، عَنْ جَبَلَةَ، قَالَ سَمِعْتُ ابْنَ، عُمَرَ - رضى الله عنهما - يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ الشَّهْرُ كَذَا وَكَذَا وَكَذَا ‏"‏ ‏.‏ وَصَفَّقَ بِيَدَيْهِ مَرَّتَيْنِ بِكُلِّ أَصَابِعِهِمَا وَنَقَصَ فِي الصَّفْقَةِ الثَّالِثَةِ إِبْهَامَ الْيُمْنَى أَوِ الْيُسْرَى ‏.‏
Reference : Sahih Muslim 1080l
In-book reference : Book 13, Hadith 15
USC-MSA web (English) reference : Book 6, Hadith 2374
  (deprecated numbering scheme)