Sahih al-Bukhari 1908
Sahih al-Bukhari · Book 30: Fasting · "Start observing Saum on seeing the crescent of Ramadan, and stop on seeing the crescent (of Shawwal)."
Narrated Ibn `Umar:
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.
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.