عَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ أَنَّهُ قَالَ: ((إِنَّا أُمَّةٌ أُمِّيَّةٌ لَا نَكْتُبُ وَلَا نَحْسُبُ، الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا))، وَعَقَدَ الْإِبْهَامَ فِي الثَّالِثَةِ، ((وَالشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا))، يَعْنِي تَمَامَ ثَلَاثِينَ
It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: We are an unlettered nation; we neither know how to write nor how to calculate. The month is of such and such days, and such and such days, and such and such days. The third time, the Prophet (peace and blessings be upon him) folded his thumb (i.e., indicating 29 days). Then he said: The month is of such and such days, and such and such days, and such and such days, meaning a complete (30) days.