The Prophet ﷺ said, "We are an unlettered nation. We do not write nor calculate. The month is like this, and like this, and like this," holding up his fingers three times, until he indicated twenty-nine.
أخبرنا محمد بن المثنى قال حدثنا عبد الرحمن عن سفيان عن الأسود بن قيس عن سعيد بن عمرو
عن النبي ﷺ قال إنا أمة أمية لا نكتب ولا نحسب الشهر هكذا وهكذا وهكذا ثلاثا حتى ذكر تسعا وعشرين
Muslim narrated it, hadith no. 1080/15b (see the previous hadith, narrated from Abdul-Rahman bin Mahdi from Sufyan Thawri), and Bukhari, in the Book of Fasting, chapter on the saying of the Prophet ﷺ, "We neither write nor calculate," hadith no. 1913, narrated from Aswad bin Qays with this same wording. It is also found in Al-Kubra, hadith no. 2450.