(11) بَابُ قَوْلِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «إِذَا رَأَيْتُمُ الْهِلاَلَ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا»
Narrated Ibn `Umar:
The Prophet (ﷺ) said, "The month is like this and this," (at the same time he showed the fingers of both his hands thrice) and left out one thumb on the third time.
Reference: Sahih al-Bukhari 1908
In-book reference: Book 30, Hadith 18
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 132