It was narrated from Saeed bin 'Amr bin Saeed bin Abi Al-As, that:

he heard Ibn 'Umar narrate that the Prophet ﷺ 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.

أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَمُحَمَّدُ بْنُ بَشَّارٍ، عَنْ مُحَمَّدٍ، عَنْ شُعْبَةَ، عَنِ الأَسْوَدِ بْنِ قَيْسٍ، قَالَ سَمِعْتُ سَعِيدَ بْنَ عَمْرِو بْنِ سَعِيدِ بْنِ أَبِي الْعَاصِ، أَنَّهُ سَمِعَ ابْنَ عُمَرَ، يُحَدِّثُ 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 .