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. (Using translation from Nasāʾī 2141)
عَنِ النَّبِيِّ ﷺ أَنَّهُ قَالَ إِنَّا أُمَّةٌ أُمِّيَّةٌ لَا نَكْتُبُ وَلَا نَحْسُبُ الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا وَعَقَدَ الْإِبْهَامَ فِي الثَّالِثَةِ وَالشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا يَعْنِي تَمَامَ ثَلَاثِينَ