Hadith 13761

Chapter 2

باب 2

(2) بَاب وُجُوبِ صَوْمِ رَمَضَانَ لِرُؤْيَةِ الْهِلَالِ وَالْفِطْرِ لِرُؤْيَةِ الْهِلَالِ وَأَنَّهُ إِذَا غُمَّ فِي أَوَّلِهِ أَوْ آخِرِهِ أُكْمِلَتْ عِدَّةُ الشَّهْرِ ثَلَاثِينَ يَوْمًا

(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.
15
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 .

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).

Reference: Sahih Muslim 1080 l

In-book reference: Book 13, Hadith 15

USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2374