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.‏

Reference : Sahih al-Bukhari 1908
In-book reference : Book 30, Hadith 18
USC-MSA web (English) reference : Vol. 3, Book 31, Hadith 132
(deprecated numbering scheme)