Sahih al-Bukhari 1908

Arabic

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.‏

English translation

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, Book of Fasting. English translation by Dr. Muhammad Muhsin Khan.