Sahih al-Bukhari, Hadith 1908 ⓘ
Narrated Ibn `Umar: The Prophet 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.
حَدَّثَنَا أَبُو الْوَلِيدِ، حَدَّثَنَا شُعْبَةُ، عَنْ جَبَلَةَ بْنِ سُحَيْمٍ، قَالَ سَمِعْتُ ابْنَ عُمَرَ ـ رضى الله عنهما ـ يَقُولُ قَالَ النَّبِيُّ صلى الله عليه وسلم " الشَّهْرُ هَكَذَا وَهَكَذَا " . وَخَنَسَ الإِبْهَامَ فِي الثَّالِثَةِ.
also referenced under: Book 30, Hadith 18 (In-Book) & Volume 3, Book 31, Hadith 132 (USC-MSA)