The Prophet ﷺ said, "We are an unlettered nation. We neither write nor calculate. The month is like this and like this," meaning sometimes twenty-nine days and sometimes thirty.
عن النبي ﷺ أنه قال: إنا أمة أمية لا نكتب ولا نحسب الشهر هكذا وهكذا يعني مرة تسعة وعشرين، ومرة ثلاثين.
Bukhari narrated it in the Book of Fasting (1913), and Muslim in the Book of Fasting (1080: 15), both through the route of Shubah, who said, Aswad bin Qays narrated to us, saying, I heard Saeed bin Amr bin Saeed, who heard Ibn Umar, and he related it. The wording is Bukhari's.