Sahih Bukhari Volume 7, Book 68, Hadith 5291

حَدَّثَنَا إِسْمَاعِيلُ بْنُ أَبِي أُوَيْسٍ، عَنْ أَخِيهِ، عَنْ سُلَيْمَانَ، عَنْ حُمَيْدٍ الطَّوِيلِ، أَنَّهُ سَمِعَ أَنَسَ بْنَ مَالِكٍ، يَقُولُ آلَى رَسُولُ اللَّهِ صلى الله عليه وسلم مِنْ نِسَائِهِ، وَكَانَتِ انْفَكَّتْ رِجْلُهُ فَأَقَامَ فِي مَشْرُبَةٍ لَهُ تِسْعًا وَعِشْرِينَ، ثُمَّ نَزَلَ فَقَالُوا يَا رَسُولَ اللَّهِ آلَيْتَ شَهْرًا‏.‏ فَقَالَ ‏"‏ الشَّهْرُ تِسْعٌ وَعِشْرُونَ ‏"‏‏

Narrated By Anas bin Malik : Allah's Apostle took an oath that he would abstain from his wives, and at that time his leg had been sprained (dislocated). So he stayed in the Mashruba (an attic room) of his for 29 days. Then he came down, and they (the people) said, "O Allah's Apostle! You took an oath to abstain from your wives for one month." He said, "The month is of twenty nine days."