Ibn Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying:

The month is thus, and thus, and thus, and he flapped his hands with all their fingers twice. but at the third turn, folded his right thumb or left thumb (in order to give an idea of twenty-nine).

وَحَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا شُعْبَةُ، عَنْ جَبَلَةَ، قَالَ سَمِعْتُ ابْنَ، عُمَرَ - رضى الله عنهما - يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏

‏ الشَّهْرُ كَذَا وَكَذَا وَكَذَا ‏

‏ ‏.‏ وَصَفَّقَ بِيَدَيْهِ مَرَّتَيْنِ بِكُلِّ أَصَابِعِهِمَا وَنَقَصَ فِي الصَّفْقَةِ الثَّالِثَةِ إِبْهَامَ الْيُمْنَى أَوِ الْيُسْرَى ‏.‏