The Prophet ﷺ said, "We are an unlettered nation. We neither write nor calculate. The month is like this, and this, and this," folding his thumb on the third time, "and the month is like this, and this, and this," meaning a full thirty.
حدثنا محمد بن جعفر حدثنا شعبة عن الأسود بن قيس سمعت سعيد بن عمرو بن سعيد يحدث أنه
عن النبي ﷺ أنه قال إنا أمة أمية لا نكتب ولا نحسب الشهر هكذا وهكذا وهكذا وعقد الإبهام في الثالثة والشهر هكذا وهكذا وهكذا يعني تمام ثلاثين
Its isnad is sahih. Aswad bin Qays Abdi, his reliability was established earlier at 921, and we add here that Bukhari included his biography in al-Kabir 1/1/448. Saeed bin Amr bin Saeed bin As bin Saeed bin As bin Umaya Umawi is a reliable tabii. He was deemed reliable by Abu Hatim, Abu Zura, Nasai, and others, and Zubayr bin Bakkar said, "He was among the scholars of Quraysh in Kufa." Bukhari included his biography in al-Kabir 2/1/456-457 and said, "He heard from Ayesha and Ibn Umar."This hadith was narrated by Muslim, 1:298-299, through the route of Muhammad bin Jafar, Ibn Muthanna, and Ibn Bashar, from Shubah. Part of its meaning has already appeared through another route at 4981, and see also 4866.