The Prophet ﷺ said: The month consists of twenty-nine days, but do not fast till you sight it (the moon) and do not break your fast till you sight it. If the weather is cloudy, calculate it thirty days. When the twenty-ninth of Sha'ban came, Ibn Umar would send someone (who tried) to sight the moon for him. If it was sighted, then well and good; in case it was not sighted, and there was no cloud and dust before him (on the horizon), he would not keep fast the next day. If there appeared (on the horizon) before him cloud or dust, he would fast the following day. Ibn Umar would end his fasting alone with the people, and did not follow this calculation.  

قَالَ رَسُولُ اللَّهِ ﷺ إِنَّا أُمَّةٌ أُمِّيَّةٌ لاَ نَكْتُبُ وَلاَ نَحْسُبُ الشَّهْرُ هَكَذَا وَ هَكَذَا وَهَكَذَا وَخَنَسَ سُلَيْمَانُ أُصْبَعَهُ فِي الثَّالِثَةِ يَعْنِي تِسْعًا وَعِشْرِينَ وَثَلاَثِينَ  

Ibn ‘Umar reported the Apostle of Allaah ﷺ as saying “The month consists of twenty nine days, but do not fast till you sight it (the moon) and do not break your fast till you sight it. If the weather is cloudy, calculate it thirty days. When the twenty-ninth of Sha’ban came, Ibn ‘Umar would send someone (who tried) to sight the moon for him. If it was sighted then well and good, in case it was not sighted and there was no cloud and dust before him (on the horizon) he would not keep fast the next day. If there appeared (on the horizon) before him cloud or dust, he would fast the following day. Ibn ‘Umar would end his fasting alone with the people and did not follow this calculation.”  

قَالَ رَسُولُ اللَّهِ ﷺ الشَّهْرُ تِسْعٌ وَعِشْرُونَ فَلاَ تَصُومُوا حَتَّى تَرَوْهُ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ ثَلاَثِينَ قَالَ فَكَانَ ابْنُ عُمَرَ إِذَا كَانَ شَعْبَانُ تِسْعًا وَعِشْرِينَ نُظِرَ لَهُ فَإِنْ رُؤِيَ فَذَاكَ وَإِنْ لَمْ يُرَ وَلَمْ يَحُلْ دُونَ مَنْظَرِهِ سَحَابٌ وَلاَ قَتَرَةٌ أَصْبَحَ مُفْطِرًا فَإِنْ حَالَ دُونَ مَنْظَرِهِ سَحَابٌ أَوْ قَتَرَةٌ أَصْبَحَ صَائِمًا قَالَ فَكَانَ ابْنُ عُمَرَ يُفْطِرُ مَعَ النَّاسِ وَلاَ يَأْخُذُ بِهَذَا الْحِسَابِ  

'Umar b. 'Abd al-'Aziz wrote (a letter) to the people of Basrah: It has reached us from the Messenger of Allah ﷺ, like the tradition narrated by Ibn 'Umar from the Prophet ﷺ. This version adds: The best calculation is that when we sight the moon of Sha'ban on such-and-such date, fasting will being on such-and-such dates, Allah willing, except they they sight the moon before that (date).  

عَنْ رَسُولِ اللَّهِ ﷺ نَحْوَ حَدِيثِ ابْنِ عُمَرَ عَنِ النَّبِيِّ ﷺ زَادَ وَإِنَّ أَحْسَنَ مَا يُقَدَّرُ لَهُ إِذَا رَأَيْنَا هِلاَلَ شَعْبَانَ لِكَذَا وَكَذَا فَالصَّوْمُ إِنْ شَاءَ اللَّهُ لِكَذَا وَكَذَا إِلاَّ أَنْ تَرَوُا الْهِلاَلَ قَبْلَ ذَلِكَ  

We kept fast for twenty-nine days along with the Prophet ﷺ more often than we kept fast for thirty days.  

لَمَا صُمْنَا مَعَ النَّبِيِّ ﷺ تِسْعًا وَعِشْرِينَ أَكْثَرُ مِمَّا صُمْنَا مَعَهُ ثَلاَثِينَ  

The Prophet ﷺ as saying: The two months of 'Id (festival), Ramadan and Dhu al-Hijjah, are not defective.  

عَنِ النَّبِيِّ ﷺ قَالَ شَهْرَا عِيدٍ لاَ يَنْقُصَانِ رَمَضَانُ وَذُو الْحِجَّةِ