he heard Ibn 'Umar narrate that the Prophet said; "We are an unlettered Ummah; we do not use computation. The month is like this, and this, and this," and he held down his thumb the last time. "And the month is like this, and this, and this," completing thirty.
عَنِ النَّبِيِّ ﷺ قَالَ إِنَّا أُمَّةٌ أُمِّيَّةٌ لاَ نَحْسِبُ وَلاَ نَكْتُبُ وَالشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا وَعَقَدَ الإِبْهَامَ فِي الثَّالِثَةِ وَالشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا تَمَامَ الثَّلاَثِينَ