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.

قَالَ النَّبِيُّ ﷺ الشَّهْرُ هَكَذَا وَهَكَذَا وَخَنَسَ الإِبْهَامَ فِي الثَّالِثَةِ