The Book of Fasting
13/4. Chapter:
The month may be twenty-nine days
Sahih Muslim
حَدَّثَنِي هَارُونُ بْنُ عَبْدِ اللَّهِ، وَحَجَّاجُ بْنُ الشَّاعِرِ، قَالاَ حَدَّثَنَا حَجَّاجُ بْنُ مُحَمَّدٍ، قَالَ قَالَ ابْنُ جُرَيْجٍ أَخْبَرَنِي أَبُو الزُّبَيْرِ، أَنَّهُ سَمِعَ جَابِرَ بْنَ عَبْدِ اللَّهِ، - رضى الله عنهما - يَقُولُ اعْتَزَلَ النَّبِيُّ صلى الله عليه وسلم نِسَاءَهُ شَهْرًا فَخَرَجَ إِلَيْنَا صَبَاحَ تِسْعٍ وَعِشْرِينَ فَقَالَ بَعْضُ الْقَوْمِ يَا رَسُولَ اللَّهِ إِنَّمَا أَصْبَحْنَا لِتِسْعٍ وَعِشْرِينَ . فَقَالَ النَّبِيُّ صلى الله عليه وسلم " إِنَّ الشَّهْرَ يَكُونُ تِسْعًا وَعِشْرِينَ " . ثُمَّ طَبَّقَ النَّبِيُّ صلى الله عليه وسلم بِيَدَيْهِ ثَلاَثًا مَرَّتَيْنِ بِأَصَابِعِ يَدَيْهِ كُلِّهَا وَالثَّالِثَةَ بِتِسْعٍ مِنْهَا .
Abu Zubair is reported to have heard Jabir b. 'Abdullah (Allah be pleased with both of them) as saying: The Apostle of Allah (ﷺ) separated himself from his wives for a month. (His wives said: ) He came to us on the morning of the twenty-ninth. Upon this some, of the people said: It is the morning of twenty- ninth (according to our calculation). Upon this the Messenger of Allah (ﷺ) said: The month. may also consist of twenty-nine days. The Apostle of Allah (ﷺ) then flapped his bands thrice, twice with all the fingers of both his hand (to indicate twenty-nine) and by the third time with nine (fingers).
Reference : Sahih Muslim 1084bIn-book reference : Book 13, Hadith 28USC-MSA web (English) reference : Book 6, Hadith 2386