Sunan an-Nasa'i 3456
It was narrated that Anas said:
"The Prophet swore an oath of abstention from his wives for a month and stayed in his room for twenty-nine days. It was said: 'O Messenger of Allah, did you not swear an oath of abstention for a month?' He said: 'This month is twenty-nine days.'"
أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنَا خَالِدٌ، قَالَ حَدَّثَنَا حُمَيْدٌ، عَنْ أَنَسٍ، قَالَ آلَى النَّبِيُّ صلى الله عليه وسلم مِنْ نِسَائِهِ شَهْرًا فِي مَشْرَبَةٍ لَهُ فَمَكَثَ تِسْعًا وَعِشْرِينَ لَيْلَةً ثُمَّ نَزَلَ فَقِيلَ يَا رَسُولَ اللَّهِ أَلَيْسَ آلَيْتَ عَلَى شَهْرٍ قَالَ ‏
"‏ الشَّهْرُ تِسْعٌ وَعِشْرُونَ ‏"‏ ‏.‏