Hadith 4:738
Ghundar narrated to us, from Shu'bah, from al-Aswad ibn Qays, who said: I heard Sa'id ibn 'Amr narrating that he heard Ibn 'Umar narrating from the Prophet (may Allah bless him and grant him peace), who said: "We are an unlettered nation; we do not write and we do not calculate. The month is thus and thus," and he tied his thumb on the third [time]. "And the month is thus and thus and thus," meaning the completion of thirty.
حدثنا غندر عن شعبة عن الأسود بن قيس قال سمعت سعيد بن عمرو يحدث أنه سمع بن عمر يحدث عن النبي صلى الله عليه وسلم قال أنا أمة أمية لا نكتب ولا نحسب الشهر هكذا وهكذا وعقد الإبهام في الثالثة والشهر هكذا وهكذا وهكذا يعني تمام الثلاثين