The Messenger of Allah ﷺ said, "We are an unlettered nation. We neither write nor calculate. The month is like this, and like this, and like this." Sulayman folded in his finger on the third gesture, meaning twenty-nine, and thirty.
حدثنا سليمان بن حرب حدثنا شعبة عن الأسود بن قيس عن سعيد بن عمرو يعني ابن سعيد بن العاص
قال رسول الله ﷺ إنا أمة أمية لا نكتب ولا نحسب الشهر هكذا وهكذا وهكذا وخنس سليمان أصبعه في الثالثة يعني تسعا وعشرين وثلاثين
Bukhari recorded it in the Book of Fasting, chapter "The words of the Prophet ﷺ, 'We do not write and we do not calculate,'" hadith no. 1913, etc., and Muslim recorded it, hadith no. 15/1080, in the Book of Fasting, chapter on the obligation of fasting Ramadan upon sighting the crescent moon, from the hadith of Shubah, through this same chain.