Hadith 1971

وَعَنِ ابْنِ عُمَرَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أَنَا أمة أُميَّة لَا تكْتب وَلَا تحسب الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا» . وَعَقَدَ الْإِبْهَامَ فِي الثَّالِثَةِ. ثُمَّ قَالَ: «الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا» . يَعْنِي تَمَامَ الثَّلَاثِينَ يَعْنِي مَرَّةً تِسْعًا وَعِشْرِينَ وَمرَّة ثَلَاثِينَ
Ibn ‘Umar reported God’s messenger as saying, “We are an unlettered people who can neither write nor count. The month is thus and thus and thus,” drawing in his thumb when he said it the third time.* Then he said, “The month is thus and thus and thus,” meaning a complete thirty days. He meant alternately twenty-nine and thirty. *Spreading out the ten fingers the first two times and nine the third time. (Bukhari and Muslim.)
Hadith Reference Mishkat al-Masabih / The Book of Fasting / Hadith: 1971
Hadith Grading الألبانی: مُتَّفَقٌ عَلَيْهِ  |  زبیر علی زئی: متفق عليه
Hadith Takhrij ´تحقيق و تخريج: محدث العصر حافظ زبير على زئي رحمه الله` «متفق عليه، رواه البخاري (1913) و مسلم (1081/15)»