Sahih Bukhari Volume 7, Book 67, Hadith 5203

حَدَّثَنَا خَالِدُ بْنُ مَخْلَدٍ، حَدَّثَنَا سُلَيْمَانُ، قَالَ حَدَّثَنِي حُمَيْدٌ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ آلَى رَسُولُ اللَّهِ صلى الله عليه وسلم مِنْ نِسَائِهِ شَهْرًا وَقَعَدَ فِي مَشْرُبَةٍ لَهُ فَنَزَلَ لِتِسْعٍ وَعِشْرِينَ فَقِيلَ يَا رَسُولَ اللَّهِ إِنَّكَ آلَيْتَ عَلَى شَهْرٍ‏.‏ قَالَ ‏"‏ إِنَّ الشَّهْرَ تِسْعٌ وَعِشْرُونَ ‏"‏‏.

Narrated By Anas : Allah's Apostle took an oath that he would not visit his wives for one month, and he sat in an upper room belonging to him. Then, on the twenty ninth day he came down. It was said, "O Allah's Apostle! You had taken an oath not to visit your wives for one month." He said, "The (present) month is of twenty-nine days."