It was narrated from Umm Salamah that: the Messenger of Allah (ﷺ) swore to keep away from some of his wives for a month. On the twenty-ninth day, in the evening or the morning, it was said: "O Messenger of Allah, only twenty-nine days have passed." He said: "The month is twenty-nine days."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَحْمَدُ بْنُ يُوسُفَ السُّلَمِيُّ، حَدَّثَنَا أَبُو عَاصِمٍ، عَنِ ابْنِ جُرَيْجٍ، عَنْ يَحْيَى بْنِ عَبْدِ اللَّهِ بْنِ مُحَمَّدِ بْنِ صَيْفِيٍّ، عَنْ عِكْرِمَةَ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أُمِّ سَلَمَةَ، أَنَّ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ آلَى مِنْ بَعْضِ نِسَائِهِ شَهْرًا فَلَمَّا كَانَ تِسْعَةً وَعِشْرِينَ رَاحَ أَوْ غَدَا . فَقِيلَ يَا رَسُولَ اللَّهِ إِنَّمَا مَضَى تِسْعٌ وَعِشْرُونَ . فَقَالَ " الشَّهْرُ تِسْعٌ وَعِشْرُونَ " .
sunnah.com