Sahih al-Bukhari 1910
Narrated Um Salama:
The Prophet (ﷺ) vowed to keep aloof from his wives for a period of one month, and after the completion 
of 29 days he went either in the morning or in the afternoon to his wives. Someone said to him "You 
vowed that you would not go to your wives for one month." He replied, "The month is of 29 days."
حَدَّثَنَا أَبُو عَاصِمٍ، عَنِ ابْنِ جُرَيْجٍ، عَنْ يَحْيَى بْنِ عَبْدِ اللَّهِ بْنِ صَيْفِيٍّ، عَنْ عِكْرِمَةَ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أُمِّ سَلَمَةَ ـ رضى الله عنها ـ أَنَّ النَّبِيَّ صلى الله عليه وسلم آلَى مِنْ نِسَائِهِ شَهْرًا، فَلَمَّا مَضَى تِسْعَةٌ وَعِشْرُونَ يَوْمًا غَدَا أَوْ رَاحَ فَقِيلَ لَهُ إِنَّكَ حَلَفْتَ أَنْ لاَ تَدْخُلَ شَهْرًا‏.‏ فَقَالَ ‏
"‏ إِنَّ الشَّهْرَ يَكُونُ تِسْعَةً وَعِشْرِينَ يَوْمًا ‏"‏‏.‏