Sahih Muslim 1083
Sahih Muslim · Book 13: The Book of Fasting · The month may be twenty-nine days
Zuhri reported that (once) the Messenger of Allah (ﷺ) took an oath that he would not go to his wives for one Month. Zuhri said that 'Urwa narrated to him from 'A'isha (Allah be pleased with her) that she said:
حَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ ، أَخْبَرَنَا عَبْدُ الرَّزَّاقِ ، أَخْبَرَنَا مَعْمَرٌ ، عَنِ الزُّهْرِيِّ ، أَنَّ النَّبِيَّ صلى الله عليه وسلم أَقْسَمَ أَنْ لاَ يَدْخُلَ عَلَى أَزْوَاجِهِ شَهْرًا - قَالَ الزُّهْرِيُّ - فَأَخْبَرَنِي عُرْوَةُ عَنْ عَائِشَةَ - رضى الله عنها - قَالَتْ لَمَّا مَضَتْ تِسْعٌ وَعِشْرُونَ لَيْلَةً أَعُدُّهُنَّ دَخَلَ عَلَىَّ رَسُولُ اللَّهِ صلى الله عليه وسلم - قَالَتْ بَدَأَ بِي - فَقُلْتُ يَا رَسُولَ اللَّهِ إِنَّكَ أَقْسَمْتَ أَنْ لاَ تَدْخُلَ عَلَيْنَا شَهْرًا وَإِنَّكَ دَخَلْتَ مِنْ تِسْعٍ وَعِشْرِينَ أَعُدُّهُنَّ فَقَالَ " إِنَّ الشَّهْرَ تِسْعٌ وَعِشْرُونَ " .
When twenty-nine nights were over, which I had counted, the Messenger of Allah (ﷺ) came to me (he came to me first of all). I said: Messenger of Allah, you had taken an oath that you would not come to us for a month, whereas you have come after twenty nine days which I have counted. Whereupon he said: The month may also consist of twenty-nine days.